DOCUMENT RESUME

ED 250 202 St 045 222

AUTHOR Chapman, Robert D.; Bondurant, R. Limn,Jr. TITLE Comet Halley Returns. A Teacher's Guide, 1985-1986. INSTITUTION National Aeronautics and Space Administration, Greenbelte, Md. Goddard Space FlightCenter. REPORT NO EP-197 . PUB DATE Jul 84 NOTE 56p.. AVAILABLE FROM of Documents, U.S. Government Printing Office, Washington, DC 20402. PUB TYPE Guides - Classroom Use- Guides(For Teachers) (052) tDRS PRICE MF01/PC03'Pluk Postage. DESCRIPTORS *Astronomy; *Elementary School Science;Elementary Secondary Education; *Science Activities; Science Education; *Secondary School Science IDENTIFIERS *Comets

. ABSTRACT This booklet was designed'aian aid for elementary and secondary school teachers. It is dividedinto two distinct parts. Part Iis a brief tutorial which introducessome of the most important concepts about, comets.Areas addressed include: the historical importance of Comet Halley; howcomets are found and names; cometary orbits; what Comet Halley will look like;how and when this comet can be viewed; thenature of comets; and Comet Halley in 1910. Part II containsa number of suggested activities built around the comet. These include bothclassrbom exercises and carefully described field work to observethe comet. Guidance is provided on where to look for thecomet, how to observe it, and how to photograph'it. Virtuallyevery exercise can be done without special equkpient; all that is ,needed some thought on the part of the teacher to adapt the activitiesto the appropriate grade. level. A list of selected readings is providedat the end of the booklet for those who desirea more in -depth treatment of the subject. (JN)

* * * * * ** * * * * * * * ******************************************************* Reproductions supplied by EDRS are the best that can.be made *. * from the original document. ************************************************ * * * * * * * * * * * * * * * * * * * * * ** VA DEPAimynNTcoEDUCATION, NATIONAL INSTITUTE OF EDUCATION EDUCATIONAL RESOURCES INFORMATION CENTER (ERIC) This ocume has been reproduced as recer ad fro the person or otganizetion orig. mg it

IIMinor changes have been made to Itnprol reproduction quality

Points of view or opinions stated in this docu mot do no') necessarilyrilPraniit Off iCialNIE POSRIOn Or policy.

"PERMISSU'N TO REPRODUCE THIS MATERI HAS BEEN GRANTED BY

TO THE SpUCATIONAL RESOURCES INFORMATION CENTER (ERIC)." 2 C

St 4

7`

COVER: Computer enhanced image of Comet Hefty from May 26, 1910. The image is the sum of four photographs originally made at the Helwan Observatory inEgypt. The photographs were digitized, added together and computer enhanced at the Interactive Astronomical Data Analysis Facility of the Goddard Space Flight Center by Dr. Daniel A. Xlinglesmith, Ill. 3 4

4

It 11.

3 COMET HALLEY RETURNS

I. O A Teachers' Guide 1985-1986

f. t

44

t

'4t National Aerqnautics and Space Administration

Educational Programs Office of Public Affairs Goddard Space Flight Center Greenbelt, Maryland 0771.

0

Robert D. Chapman, Ph.D. Associate Chief, Laboratory for Astronomy and Solar Physics Goddard Space Flight Center Greenbelt, Maryland and For Sale by the Superintendent of Documents, R. Lynn Bondurint, Jr., Ph.D. U.S. Government Printing Office, Educational Services Officer, Washington, D.C. 20402 Office of External Affaiis Lewis Research Center EP-197 Cleveland, Ohio )1-

,4 4.

O

'or

PREFACE A

. . Comet Halley is on its way to a 1985.86 rendezvous with the earth and the sun. This most famous of all comets is sure to generate an increasing level of interest among 'the general .public and particularly among young people as it 4 draws ever nearer to us, The event offers an unparalleled learning opportunity for students at all ,levels to gain the skills, understanding and enthusiasm necessary to study science. T his booklet has beenput together as an aid for teachers in elementary and . secondary schools.It is divided into two distinct parts. The first part is a brief tutorial which introduces some of the most important concepts about comets, ,including their historical significance. In the limited space available, it Om only hit the high points. A list of selected readings is provided at the end of the 1;soklet for those who desire a more in-depth treatment of the subject. The second part of the booklet contains a number of suggested activities, built around the comet.These ,include both classroom exercises and carefully described field work to observe the comet. Guidance is provided on where to look for the cornet, how to observe it, and how to. photograph it.Virtually' every exercise can be done without special equipmentAll that is needed is some thought on the,part-01 the teacher to adapt the ictivities to the appro.,. 144, 'y priate g"rade level. Both authors of the booklet have proven, outstanding abilitiett to communicate science to laymen. In addition, Chapman is a recognized authority in the field of cometary research, having co-authored one of the few professional level textbooks on comets.The result of their collaboration is a scientifically accurate, and well-planned guide. If you use it well, your students will have a profitable educational experience with lifelong rewards.

Elva Bailey Educational Programs Officer Goddard Space Flight Center July 1984

4

4 5

o .

0 a4 a 4 CONTENTS II%It:

Page PART I. TOE NATURE OF COMETS 1 THE HISTORICAL IMPORTANCE PF COMET HALLEY . 1

HOW COMETS ARE FOUND AND NAMED 3

COMETARirORBITS' 5 WHAT WILL COMET HALLEY LOOKLIKE 9 HOW AND, WHENWE CAN VIEW COMET HALLEY 10 WHAT IS A COMET? 10 COMETS OF THE PAST 18 COMET HALLEY IN 1910 . 19

PLANS TO OBSERVE COMET HALLEY 19

.. . PART II. EDUCATIONAL ACTIVITIES -, 23 IN THE CLASS1656M 23 , Newspaper Accounts 11 23 ) , ...- Oral History 24 Time Capsule 24 Comet alley Artistically Speaking 24 Comets and Life Sciences - 24 . Comet Hall of Fame 24 OBSERVING/THE COMET 24 Brightness 25 Length of Comet Halley's Tail in Miles 28 Recording Observations 30 Plotting the Locatiiin of Comet Halley 30 ° COMET HALLEY'S. ORBIT.4. 33 PLACES TO VISIT THINGS TO DO 36 ' COMPUTING AN EPHEMERIS FOR COMET HALLEY 35

SELECTED READINGS 40

INTigX 41 O 4 Part I THE NATURE OF COMETS A bright comet is a spectacular object to behold. If Two of the most notable scientists of all timewere you had awakened around 4:00 in the morning in Galileo Galilei (1664-1642) and IsaacNewton early March, 1976 and braved thelate winter cold (1642-1727). These twomen, more than anyone yqu would have been rewarded witha beautiful else, in*nted the., modern. science of sight:a view. of the naked-eye Comet West. mechanics. It Newton also postulated the existence ofuniversal would have been visible in theeastern sky, near the gravity, which asserted that all bodies inthe uni- point where the sun would rise ina few hours. The verse attract one another. 'It is the intense gravity comet's head would have beennear the horizon, of the massive sun that holds thesolar system and its tail would have beenseen streaming upward together. toward the zenith.For those of us whosaw the comet, it will remain a memorable event. Forthose In parallel with all of these developmentsin the who have never seen one, it is well worththe effort concepts of planetary motionwere developments to observe one. in our understanding of comets.In the Fourth Century B.C., Aristotle believed that cometswere a Comet Halley will .be visible to smalltelescopes, phenomenon of the, earth's atmosphere. binoculars and even the naked In his eye in late 1985 and treatise Meteorologioa he asserts that cometsare early 1986. The event is alreadystirring interest "exhalations" in the outer reaches of the both. in the scientific community, and atmo- in the general sphere.This view was repeated by suchgreat public. What is it about this cometthat generates philosophers as Ptolemy.It is interesting to note so much interest? How can we observe it when it that the Roman Stgico philosopher LuciusSeneca paisei near the earth? What willscientists around (4 B.C.- A.D. 58) held the view that comets are the world try to find out? Thesequestions and celestial bodies which travel throughspace in others will be answered in the followingpages, as elongated orbits. Before we Ode Senecatoo much we tell the story of the comet. To begin letus take of a pat on the back,we must realize that his idea a look at the reasons that Comet Halley isso was as much of a guess as was that of Aristotle., It important historically. was Snot until the 16th century that Tycho, with his very accurate instruments, could makeobserva- tions which established that cometsare celestial THE HISLORICAL IMPORTANCEOF objects. He observed the position of COMET HALLEY a bright comet' that appeared in 1577 from various sites inEurope., If the comet were in the earth's atmosphere, To tell the whole story of the importance then of Comet it would have a measurable parallax (see Figure1), Halley would take a large volume, forwe would that is, it would shift against the backgroundof the have to look at the whole history ofthe develop- stars by a measurable amountas he moved hilt ment of our understanding the nature ofthe equipment about on the surface of theearth, planets and how they move.The early Greeks viewing the csmet com different angles. He could thought that the planeall revolved around the not detect a mbasura e parallactic shift, earth in orbits that so he con- a combinations of perfect cluded that the come had to be at leastseveral circles. In the.first cen A.D., Claudius Ptolemy times farther away from ttie earth than themoon: proposed a complex sys of epicycles that ex- whose parallax he could measure. plained almost all of the observed factsabout the motions of celestial bodies. This systemwas used In 1665, the Great Plague closed down Cambridge to predict the motions of the planets for althost 14 University, and the 23-year old Isaac Newtonwas centuriesa fact that attests to thesuccess of the forced into a two-year hiatus in his formal .ate theory. studies. However, natural philosophers began to With few responsibilities, Newton had littleto do take a new logk at the universe in the Renaissance. other than contemplate the myiteries of theuni- Nicolas Obpdhicus (1473-1543) assertedthat the verse. The results of his contemplationswere' planets all revolved about thesun.Actually, formidable. He arrived ata formalized system of Copernicus' novel idea was basedmore on aesthetic mechanics and a law of gravitation thattogether grounds than on the basis ofany scientific results. put the study of motions in the solar systemon a However, that would change in the late16th cen- firm mathematical footing. tury, when Tycho Brahe (1546-1601) developed new instruments that permitted him to measure Among his other accomplishments, Newton fount the positions of celestial objects withunparalleled a way of calculating the characteristics of the accuracy, before the invention of the telescope. orbital paths of a comet froma eerier of observa- Working with Tycho's data, Johannes Kepler tions of the comet's position in the sky. Hedid (1561-1630) showed that Marsmoves. around the this, in part, by. assuming initially that thecomet's sun in an elliptical orbit. He then went one step Orbit was parabolic in shape. Ayoung friend and farther and hypothesized that all planets orbit the contemporary, Edmund Halley, applied the method sun in elliptical orbits. to a series of comets thatwere observed in the

1 COMET

4

(.#

SMALL \ PARALLAX ANGLE

LARGE PARALLAX ANGLE

I

ASSUMED POSITIO OF COMET

If the comet were in the atmosphere, it would appear If comet were farther than the moon, the shift due to to shift against background stars due to a small the motion of the observer would be smaller, and motion of the observer. ° would have been difficult to measure in 16th century.

Figure 1. A comparison of the parallax of a comet if it were In the earth's atmosphere and if itwere beyond the moon.

2 1 8 S.

14th through the 17th centuries.,Among the sur- comet was well over a billion miles from the,sun, prises that he turned -upwas the fact that comets farther away than the planet Saturn.It appeared observed in 1531, 1607, and 1682 all hadvery as a, very faint, starlike point of light on the photo- similar orbits. He concluded that thethree comets graph (Figure 3); it was too far from thesun to were, in fact, repeated appearances .ofone and the have formed the features that normally charac- same comet that orbited around the sun once every t/terize a comet. How, then, did. theastronomers 75 years in an elongated orbit,as shown in Figure recognize it as a comet? There are tworeasons: 2. He predicted that it would return to the.vicinity first, the starlight point of lightwas seen to move of the earth and sun in .1758. The comet passed relative to the stars when twoexposures taken Bev- perihelion the closest point in its orbit to the .eral hours apart were intercornpared; and,scien- sun-- on March 12,1759, after being recovered by. tists had calculated where the comet oughtto be an amateur astronomer on December 25, 1758. using observations made at All thepasses since Unfortunately, Halley died befdre the cometre- Edmund Halley's days, and the cometwas found at turned, and he did not see it again: Thereturn the correct position. providedincontrovertableproof of Newton's theory, and Halley's orbit calculations.In honor of the great importance of Halley's prediction,the HOW COMETS ARE FOUND AND NAMED comet was named after him. Many comets have been discovered byamateur In October 1982, astronomers working withthe astronomers who spend hours sweeping the sky great 5-Meter (200-inch) reflecting telescopeon with wide field,low magnification' telescopes called Palomar Mountain in California obtained thefirst "comet seekers."Typically, the observersees a observations of Comet Halleyas it proceeded faint, fuzzy object among the stars insome region toward a 1985-86 rendezvous with thesun and the ,of the sky, where catalogs'say there should not be earth. When these early imageswere obtained, the a fuzzy object.There are many types of objects

A

. .

PLUTO TO 'VERNAL! EQUINOX

NEPTUNE

URANUS

. SATURN HALLEY

Fi',ure 2. The orbit of Comet Halley. The circles represent, from theInside out, the orbits of earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The outer three planets orbits' are added for reference. These planetswere unknown at the time Halley ascertained the parameters of the ortoit of Comet Higley.Note that in this picture the planets orbit ina counterclockwise direction, while the comet orbits in a clockwise direction. The vie*. isfrom above the north pole of the earth.

3 A

. T. dr_

x

-

is .. _ =-:-:'F-F--L.- ."'-"."-.7..------=-'...... -.Vr. '.... -- - .2:.. 7: .1.=:''."'' - - - -... ,_ ---.;1.----.2Z--4_,..._ ../-*--_-7--.----.---. .--..:_----7:-."-- .

T..=--.._-1---..._=------r---,...,..--.._..__,..,:: ...... :--....--.=7.7..:="7.....:-"Za.- - 727. 7... . . _ ...... "--...2.. ---all ----. ....---.X.:',./..r.: ..1.1:J.F_ - i,..._ 7-2-.7=7 ...a= .....2-::. _ - r_. :_....-rire4--. -,7 - .=----_-.ir -=' __71_-.7-ii .s.7. 4.4-

t...... -7..::-.----j--.ix....Ix:ji:Izisz...-_:-,-; 2:-;::_---::-. -

lC . _ , .. I7 : . . -. : T - - ,.. : 4 . s , - - r. . as-g, l'r414stiff

Figure 3. Rediscovery photograph of Comet Halley made in October 1982 using thegreat 200-inch reflector on Palomar Mountain, California. (Courtesy of G. Edward Danielson of Ca/tech.) that appear fuzzy to a small telescope:galaxies, Not all comets have been found by amateurs, of star clusters and glowing interstellar gas clouds all course, although comet seeking is one field where do.The clue as to whether an observed ftizzy. the layman can assist and even compete with the object is a comet or not is its motion. A comet professional. The most successful comet seeker of will be observed to move its p9sition over the all times, Jean Louis Pons (1761-1831) started his course of several hours, while any of the other career in astronomy as the caretaker at the Mar- objects mentioned do not appear to move relative seilles Observatory in 1789. In his active lifetime, to the stirs. he discovered over. thirty-seven comets and worked his way up in the astronomy profession until he was appointed director of the Marna Observatory. When a person discovers a comet he or she is re- warded by having the comet named after him or her. Occasionally, two or even more observers find One of the most incredible stories of the discovery the same comet nearly simultaneously, in which of a comet is the following. An astronomer at the case up to three independent Observers will hive Lick Obiervatory. D. Perrin, discovered a comet their names attached to the comet. For example, in 1896. On subsequent nights he continued to Comet Ikeya-Seki, which became visible to the make routine observations of the object. At one naked eye in 1965, was discovered by two Japanese point, he received a telegram from the Kiel Observa- amateurs, K. Ikeya and T. Seki. Kaoru Ikeya dis- tory also reporting the position of the comet. How- covered his first comet in 1963 when he was only ever, unknown to him, there had been an error in the 19 years old, using a home-made telescope.His transmission of the telegram resulting in an incor- discovery, which culminated 16 months of search- rect position being listed. The incorrect position was ing, brought him much deserved fame for his only two degrees from the correct one, so Perrine energy and persistence.He has now discovered did not notice the difference. When he looked at many more comets. the wrong position, there was a second new eomet.

4 10 Comet seeking is an area wherewomen have dis- An ellipse is a closed figure obtained' by inter- tinguished themselves.Carolina Herschel discov- secting it circular cohe with a plane. As shown in ered or co-discovered eight comets between 1786 FigUre 5, when the intersecting plane isperpen- and 1797, earning hera world wide reputation. dicular to the axis of the cone one obtains the In the U.S., Maria Mitchell discovered herfirst limiting case of a circle. When the plane is tipped comet, Comet Mitchell, in 1847: She wenton at an angle to the axis, one obtains an ellipse. The to become a professor of astronomy at Vasser greater the intersecting plane is tipped, themore College. Mitchell is considered to be the first the ellipse is elongated, until a point is. reached American women astronomer. . where the curve resulting from the intersection is no longer a closed curvq. Wheh the plane is parallel The method of naming comets after the discoverer to a generatrix (a straight line through theapex of leads to some tongue twisters.Names such as the cone and on the suktace of thecones for exam- Comet Honda-Mrkos-Pajdusakova takea while to ple, a pole in a tepee is V,generatrix) of thecone, spell out. The Astronomers A. Schwassmannand the figure is a parabola. If the plane is madepar- A. A. Wachmaun together have discovered several allel to the axis of the cone, then the conic section comets, so we must speak of Comet Schwassraann- 'generated is a hyperbola. Wachhiann I, Com Schwassmann-Wachmann II and so forth.In 1 73, L. Kohoutek discovered The size and shape of the comet's orbit and the two comets within eight day period. The first, orientation of the orbit in spaceare specified by and fainter Comet KOhoutek passed perihelion (its six quantities known as the orbital elements of the closest point to the inin)a few months after dis- comet. Once the orbital elements are ascertained, ,covery and faded rapidly from view. However, the a table of the future positions called an ephemeris second one became quite bright and stirreda lot of can be computed.An initial calculation of the interest at the time. Pigure 4 isone picture Made orbital elements can be carried outonce three of the second Kohoutek durihg 1974. Bothcomets accurate measurements of the comet's position were called Comet Kohoutek, a fact that cduld hate been made. Usually, only a smallarc of the lead to some confusion. ,orbit is included between the three initial posi- tionsrand the calculation of the elementsof the To avoid this type of confusion, cometsare num- .whole orbit can be inaccurate. An ephemeris cal- bered as well as named. Thereare two numbering culated from the initial orbital elements is usually systems. First of all, when a comet is found, it is only a rough approximation to the expectedcom- designated according to\the order of discovery. The etary motion. However, as more position observa- first comet discovered in 1983 is termed 1,983a, tions become available, the orbital elementscan be the second 1983b, and so on. Comet IRAS-Araki- successively refined, and more accurate ephemer- Alcock was the fourth comet discovered in 1983, ides can be calculated. For the initial calculation so it is called comet 1983d.(Incidentally, this of the elements of the orbit ofa comet, astron- latter comet was first noticed in data gathered by omers often follow Halley's lead and assume that NASA's Infrared Astronomy Satellite (IRAS),ao it the comet moves.in a parabolic orbit. In fact, ina was named in honor of the satellite, ,a notable ex- large fraction of the cases the refined orbit turns ception to the rule of naming comets after human out to be very close to a parabola. discoverers.)On the scheme based on year and orderof discovery, the two Comets Kohoutekwere One does not have to unders ,d about orbital ele- comet 1973e and 1973f.The second comet- ments to understand comets. however, let's takea numbering system is based on the order in which closeillOk at thiconcept fpr those of you'whoare comets reach the perihelion points in their orbits. interested. There is more Thanone valid set of or- The first comet to pass perihelion in 1983 is bital elements, but we will chooseet particular set termed comet 1983 I, the second comet 1983 II, for discussion, First, it takes three angles, i, S2, and etc. On thissystem, Comet Schwassmann- ,to describe the orientation of the orbit inspace. Wachmann Iis also called comet 1925 II and These angles are illustrated in Figure 6. Next,we Comet Honda-Mrkos-Pajdusakovais also called must describe the size and shape of the orbit. Tra- comet 1945 III. . ditionally, the shape is given by the eccentricity,e, of the orbit. A circle has an eccentricity of 0.0, and a parabola has an eccentricity of 1.0. -Ellipses COMETARY ORBITS have eccentricities between 0.0 and 1.0. The orbit's size is given by the parameteil q, which is the di The orbit of a comet is a conic section, that is,an tance from the sun to the comet at perihelion. Te ellipse, a parabola, or a hyperbola. If the orbit is f;e elements i, SZ, w , e, and q fully describe y an ellipse, the comet will return periodically to the cometary orbit.Finally, we must specify where vicinity of the sun. A comet ina hyperbolic orbit, the comet is in the orbit. This is usually done by on the other hand, is not bound to the" solar sys- giving the time when the cometpasses perihelion. tem. Such a comet will zip by the sun and head The time of perihelion passage, T, is the sixth off into interstellar space never again to return. orbital element.

11 5 "1

.

b , ,

I,

4

411.

I

,

t * . Ak Ir .0 411

(.1

C Q .00

12 Figure 4." ComeKohoutek on January 13, 1974. (JointObservatory for Cometery Research photograph.) 13 (b)

GENERATRIX

(c) (d)

Figure 5. (a A circle, Kan ellipse, (ca parabola, gnd (d a hyperbola resulting from the intersection of a cone arid a plane.

14 I

Figure 6. Graphical illustration of the angular elements of an orbit. The symbol marks the direction to the vernal equinox. The elements are explained in the text.

1

, . I Coinet Halley is a short period comet.Like the In contrast toe flat, regularly moving system Of planets, it moves, in an elliptical orbit. However, planets, thestem of comets is a hodgepodge; there the resemblance to 'planetary orbits ends. particularly1 r thet long period ,comets. The in li- While the orbits of the planets are nearly circular, nations of it orbits to the ecliptic can be y- the orbit of Comet Halley is an elongated ellipse. -thing froritt-4 to 90° and their orbital motion an

The solar system, as defined by the region of space be direct Or retrograde.Comet Halley, for,in- where tie planets move, is a flat system. With the stance, moves in a retrograde sense in an orbit tiiut exception of Pluto, rr. of the planets' orbits is is tipped by:18°o the earth's orb: tilted by more that. 4%0 degrees from the plane , of the earth's orbit. in addition to the flatness of A t the solar system, there is also a common direction' The long/periodcomets are those courts whose to almost all motions in the system. For instance, orbital revolution periods exceed 200 Years. They all the planets revolve around in a counterclock- are the majority of comets; only about 20% pf all wise direction as seen from far above the earth's comets with well observed .orbits (or about 100 north tole.In addition, all the planets except comets) fall into the short period group. In fact, Venus and Uranus rotate in the same direction that all comets that have welestablished periods are they revolve, and of the 32 planetary satellites short period comets. The ,xact size of a very ellip- known, before the last flyby of Saturn, 21 revolve tical orbit, and therefore the period of revolution about their parent planet in the counterclockwise, of the comet, is difficult to ascertain even from a or direct sense. series of good observations. I 8 .15 The group of s short period comets hasa mean (Figure 7) using data obtained by the Infrared period of revolution of sevenyears. The orbital Astronomy Satellite, as we mentioned earlier. planes of these comets ....nd to benear the plane of When it was first seen, it wasa nondescript fuzzy the ecliptic, and the comets motions tendto be object. In mid May, the comet reached nakedeye direct.Comet Halley is somewhat exceptional, brightness, when it appeared to bea nondescript, with its retrograde motion. Its period isamong the bright fuzzy object.The fuzzy spot of light is longest of the short period cometsas well. The called the coma. As the comet approaches thesun, shortest period comet known is Encke'scomet the coma will grow in size andbrightness.Typical with a period of 3.3 years. coma diameters range between 19,000 and 190,000 kilometers (12,000 and 120,000 miles).When One of the most uncertain facts aboutany comet is Com4 Halley was recovered in late 1982 (see :c the 'magnitude that it will have when it is at its Figure 3), it was so far from thesun that its coma r. brightest.The factors determining tne brightness had not yet formed.All that we see then is the are:(1) \the distance from the sun to the comet, nucleus of the cow t. Even in the brightest comets, which tells how much light reaches thecomet; the, nucleus remains a starlike point `of light. When (2) the size of the comet, which determineshow comets are near the sun the nucleiare difficult or much light it produces; and (3) the distance from impossible to see inside the brightcoma. Cometary the comet Aso the earth, which tells how' muchof nuclei are estimated to be a few kilometers in the, reflected light we receive. Of these threefac- diameter.From its brightness, for instance,we. tors, the second is the most uncertain. estimate the nucSus of Comet Halley is about 5 kilometers in diameter.The nucleus and coma make up the head of the comet. WHAT WILL COMET HALLEY LOOK LIKE The most spectacular characteristic ofa comet is When a typical comet is first ob;erved, it isa fuzzy, its tail, which develops as itnears the sun. A nearly round objecti,the sky. In May of 1983, comet could have a tail as long as 160inion kilo- astronomersfound comet 1RAS-Araki-Alcock meters (100 million miles), althoughtypicallengths.

, 4 , .4.,.

. ,,,i,4-cl.,, ; , ye,.144: ..4vw ,o',$. .10' ,4v4',4.;`, ',.4 44

, ) r .1, 1. & +`'f itL 4 .'...4.4." t. ' ., ?VV.,_.,,,,,',1 r .

c4Vr..4; '

1)f,

jit :: 4,

Fiore 7. Comet IRA3Areki-Aleockphotographed onMay 12,1983. (Joint Observatory for Cometary Researchphotograph.)

9 are more ,like a few million kilometers. A comet's .much? as 30° above the horizon:. However, at this tail. always 'points away from the sun.Come time it will once again require I, binoculars to be IRAS-Arsild-Alcock was unusual in that it did n seen. Figures 9 throtigh n summarize the discus- develop a conspicuous tail. Most comets do develo sion in pictorial form. tails.Comet Halley developed a tail each time it has been observed in the past, and there is no rea- son to believe that it will not develop one this time.,.WHAT IS A COMET?

Figure 8 is a well known photograph of Comet ok Halley made during 1910. In- the early 1950's, De F. LWhipple of the Harvard College Observatory presenteda picture of comets that, with some minor modifications, is HOW AND WHEN CAN WE VIEW peCepted today. Whipple propoied that the nucleus COMET HALLEY /is in effect a dirty iceberg, a largemass of frozen p. water, methane, ammonia, carbon' dioxide, and We need to stress at the outsetsthat the 1985-86 other constituents, in which is embedded meteor- return of Comet Halley will not be as spedtacular like, solid particles of various sizes.. When the

as the returns of 1835 or 1910. However, it will be nucleus is heated by the radiation ofthe sun, its 4 a naked. eye object during some title 'periods in ices\ sublime that is, go from the solid state to 1985 and 1986. Many of you may remember that the gaseous state and as a-result/the nucleus is Comet Kohoutek was supposed to be anunusually surrounded by a cloud of gas and the dust particles, bright comer fn late 1973 and early 1974.Yet, that were relearned. This cloud is the coma. Figure many people did not see it. Why? The experience 42 illustrates the parts of a typical comet thatwe of some of the comet observers I know is instruc- will describe fiere. :* tive. One of my colleagues travelled to a tell sit- uated comet observatory (The Joini Observatory When one turns a 'spectrograph on the eoma ofa for Cometary Research) in southern New Mexico, comet, the spectrum is found to contain lines or on a high mountain near Socorro. During.iltnuary bands which indicate the presence of simplecon- 1974, he saw the comet as a bright naked eye stituents such as H,OH, 0, CN, Cg, Cs, CO *, NH, object almost every night. By contrast, I remained NH2, CH, N2* H20+ as well at other constituents. in the Washington, D.C. area, and never reallysaw When , comets are observed at radio wavelengths, it well. The difference was the environment.' In evidence of molecules such as methyl cyanide New Mexico, there was no indAtrial air pollution (CH3CN) and hydrogen cyanide (HCN) may be and no large cities with their bright lights to drown found. With only a very few exceptions, thesecon- out the-subtle. light of the comet. In the North- stituents will not exist as such when frozen in the ...ekstern U,S.t by contrast, it is 'hard to find a;site nucleus.Instead,' they must arise' from chemical away from air pollution and city lights.You changes to the frozen molecules that are found should remember this as you plan to see Coniet there. We refer to the molecules frozen in the nu- Halley. A trip to the moinitains (and I don'tmean cleus as "parent" molecules, and the observedcon- all the way to the Southwest) will pay off in your stituents as "daughter" molecules.Astronomers ability to seethe comet.. believe the parent molecules to include ofetinary water (H20), ammonia (NH3), methane (CH4), as There are two Iiine periods when Comet well as C2, N2, and CO2. will the most easily observed by sonone in the U.S. The first period encompasses th t week in When the parent molecules are exposed after sub- November and the first week in Devember 1985, liming from the nucleus, the ultraviolet photons when the comet will be fairly bright though it from the sun. canbreak them apart (photo- will probably be obseriable only with ,binoe dissociate them) into the daughter molecules. The The comet will then pass through perihelion, and process takes place so quickly that we do not have be invisible behind the sun until about March 1986. time to see the parent molecules. The solar wind It is then predicted to be its very brightest in late particles also have an effect on the constituents. March and early April of 1986. In late March, the The complete story is far from fully understood. comet will be observable in the morning sky, We hope a detailed study of Comet Halley will toward the'southwestern horizon. If yqu are at the shed additional light on these problems. In 1973, latitude of New York City, you can see it about we issued a booklet like this one for Comet 10° above the horizon. The farther youthyou are, Kohoutek, and we ended this section by saying: the higher it will be in the sky at that time. From "we hope It stndy of Comet Kohoutek will shed southern Florida or Texas, the comet will be further light .hese problems." It did. We first almost 30° above the horizon. The best views will observed H2O* in Kohoutek, giving us additional be from the southern hemisphere.If you could evidence for the presence gf water in the nucleus. visit South America, for instance you would have But Kohoutek did not solve the problem, and an exceptional view. The comet will then swing Comet Halley will not solve the problemcom- farther north, and will be visible in the evening sky pletely either.Employment is assured for future for observers at the latitude of New York City as generations of comet scientists..

10 : 1 7.

a

1

1

I 1 I I I T 1 1 1. 1 I I I 1 1

40° N. LATITUDE aL 01.1I 30 04' COMET IN EVENING SKY JAN 5 (5.7) O , 25 % oriAPR 25 (5.4) .11=11 % 25 0 I I 20 / JAN 10 (SA) O . 20 I %

5.415 atJAN 15 (5.1) 0 COMET IN 115 MORNING SKY Z /*APR 20 (OH O Y. MAR 16(4.5) - ..... / AAR.% r ' ""*. JAN 20 44.$) 10 , / ..... (4.3)/ ... 4 APR 18 (4.6) APR (4.0). MAR 6 (4.5) / 5 IAA / / ,f/ Ell JXN IS (4.6)

110 120 130 149 150 160 170 180 190 200 210 220 230 240 250 260 270 280 SOUTH WEST AZIMUTH, DEGREES

Figura 9. Comet Halley as observedin 1986 byan observer located at 40° north latitude. The comet positions are given for the beginning ofmor twilight or at the end of evening twilight. Approximate visual magnitudesare given in parentheses following dates. (From Cornet Halley Handbook, Courtesy D. K. Yeomans.)

4 21 20 45 1111111%IIII11"111111 45 40 OEM 30°N. LATITUDE APR 29 (5.9) 40 COMET IN rit APR V (5.9) EVENING SKY 35 35

2 Alp? APR 25 (5.4) 30 30 0 ik JAN 5 (5.7) N 0 APR 23 (5.2) 25 § 25 APR 21 (4.9) / tJAN 10 (5.4) 20 COMET 1N 4--- MORNING SKY `MAR 31 (4.1) 20 MAR 21 (4.4)

Z 15 111111'11AP'It19 (4.7) 0 //MAR 16 (4.5) APR $ (4.0) JAN 15 (5.1) 15 4t MAR 11 (4.5) tat> 10 MAR 6 (4.51 \ 7s. 10 kJAN 20 (4.81 1111114APR 17 (4.5) MAR 1 (4.4), API 10 (4.0) 5 .--5 FES 24 (4.3) \ / 111\i! 11)JAN 23 (4.5) 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 EAST SOUTH WEST AZIMUTH, DEGREES

Figure 10. Comet Halley as observed in1988by an observer located at 30° north latitude. The comet positionsare given for the beginning of morning twilight or the end of evening twilight. Approximate visual magnitudesare given in parentheses following dates. From Comet Halley Handbook, Courtesy D. K. Yeomans.)

1

22 23 5 I jp APR 30 (6.0) I I I I i 1 I r 50

45 COMET IN EVENING SKY 20° N. LATITUDE 45

40 114 APR 25 (5.4) es.40 1

OWL 35

OZ 30 _44 MAR 31 (4.11 JAN S (S.7) ... MOW N ... 30 MAR 2) 14.41 l ... 1 ,, 5 ...11114(iAPR 20 (4.8) APR MAR 16 (4.5) . -25 /il I . . , / I N \ ikiJAN 10 (5.4) \ A .. 220. MAR 11 (4.5) j I I . -00 z I s. I 20 / 1 0 / 1 \ / 1 \ -- I ti 15 mAR 0 (4.5) \elAIR1110111(14°.0) iJAN15 15.1) \ --15 4 \ . 1 \ , 1 mAR 1 (4,4) i 10 I \ 1 10 \`A"COMET IN JAN 2014.8) i 4 vdliP MORNING SKY 1 .._ I 1 4. 5 \ 1 I I APR 15 (4.3) 1 , 1 n , 1 is I I 1 _I 1_(APR IS 14.31 1 1 1 j JAN 2? (4.5) I I f 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 SOUTH WEST AZIMUTH, DEGREES

Figure 11. Comet Halley as observed in 1986 by an observer located at 20° north latitude, Thecomet positions are given for the beginning ot morning twilight or the end of evening twilight. Approximate visual magnitudes are giverein parentheses followingdates, (From Comet Halley Handbook, Courtesy D. K.Yeomans.)

24 r 4

cigarette smoke in a breeze. That tail is composed of ionized molecules blown out of the coma by the solar wind, a stream of ionized atomic particles constantly blowing away from the sun. A color- picture of a comet shows the dust tail to be, yellow. ish, which is the color of sunlight ,reflected from the small particles.The gas tail is blue, on the other hand, causes by characteristiC emissions ft of the ionized molecules present (predominantly CO+ ). As the comet approaches the sun, the coma is ob- served to grow.-Clearly, this growth occurs, because of increased sublimation of the ices of the nucleus. However, *a point is reached when .the coma may actually shrink as the comet approaches even cloier to the sun. This shrinkage may occur when a point is reached when gas and dust are., blown into the tailfaster than it sublimes from Arhe nucleus. If an expert had been asked before 1969 to de- scribe a comet, he would have tokt you about, the nucleus, coma and tail.However, in 1969 and 1970 an unexpected discovery was made when the Orbiting Astronomical .0kae, reatory, 0A0-2, was turned on Cometr,,Tago-Sato-Kosaka .and Cornet' Bennett.Each was found to be surrounded by a tenuous but giant cloud of hydrogen gas. The ob- iservations of comet Bennett were subsequently verified.bylthe Orbg,geophysical Observatory. The hydrogen. cloud around.Comet TagQ -Sato Kosaka was as big .46 the sun, Mid the .loud around Comet Bennett was even_ largel. It is believed_ that the hydrogen cloud arises when ultraviolet photons from the sun.break up water molecules sublimed from the-nucleus, producing hydrogen and free OH radicals. What happens to the material blown out of the comet's nucleus? Each dust particle circles the sun in an orbit similar to the parent comet's orbit. Eventually the entire path of the comet is outlined withdust..'Occasionally, the earth passes not merely through the plane of the comet's orbit, but Figure 12. Artist's conception of a typical comet. across the very path of the comet. Then the dust particles make their presence known as a showei of meteors (shdoting stars). The particles are burned up in the earth's atmosphere due to the heat gen- According to scientific theory, alight beam is com- erated by friction between the air and the particles posed of a stream of particles called photons. A which may speed through the atmosphere as fast as beam of photons bouncing (reflected) off one side 45 km/sec. For instance, the earth crosses the orbit of a small dust particle exerts a force on it, and if of Comet Halley in May and again in October, each the dust particle. is small enough the photonscan year, and each time a meteor shower is observed. push it along.The intense sunlight falling on The Eta Aquaril shower occurs in May and the minute clustiparticles in the coma of a comet, when Orionid shower' in October.At the 1910 return the comet is near the sun, pushes the dust particles the earth was to pass through the tail of Cornet out of the coma, producing the comet's tail which Halley. Some panic was generated by the an- points away from the sun (Figure 13). If you look nouncement because of the noxious gasses in the at Figure 14 you will notice two ttti!FOne tail is tail. However, only the usual meteor shower gently curving and appears smooth. This tail is the resulted. dust tail, caused lal the mechanical action of the solar radiation.The other tailis more nearly In some cases, the debris is bunched up in a clump straightand has a turbulent appearancelike which moves around the comet's orbit.In these 25 15 tj

Figure 13. As a comet passes around the sun its tall swings around and always points away from the sun.

cases when the earth crosses the orbit only a few shower was observed. The period of the comet in meteors are observed while at other timesa spec- question is 33 years.In this case the debris is tacular meteor show is observed. 1111866 the earth highly bunched up. crossed the orbit of Comet 1866 I, anda meteor shower (the Leonid shower) with a rate of 100,000 Since cometary nuclei slowly sublime while the meteors per hour was observed.As it turns out, comet is in the vicinity of the sun, it is clear thata the earth had also passed through the comet's comet must have a finite lifetime. Some estimates orbit and met a dense shower in 1833, but in say that a comet cannot survive more than a few years between 1833 and 1866 no spectacular hundred close approaches to the sun.

16 2 AO. f

k.,:

-..

"

Figure 14. Comet West on March 9.1976. Note the gently curving, smooth dust tall uppermost in the picture and the straight bturbulent looking plasma tall (Joint Observatory for Cometary Research photoriph.) 27 28 We have arrived at a mystery. Ifa comet can last during which time it appeared to bea fairly ordi- only a few hundred passes around thesun, then nary comet. A few days after the comet was picked Comet Halley should live a few times 7500years. up on its return in 1846, it actually split into two Even a comet like Comet Kohoutek witha period distin t omets. For several months the two pieces estimated to be around 100,000years will disinte- follow e another in almost the same orbit, grate in something like 10 millionyears. Both of but wione trailing the other by 250,000 km these times is short compared to the 4billion year (160,000 miles) or so. Each piece hadan observ- age of the solar system. One would think, if this able nucleus, a coma anda tail; in short, each was a theory is correct, that. there would beno comets complete comet.However, they underwent re- left in the system today. Why dowe see comets at markable changes in brightness, with firstone then all?Particularly, why do we see short period the other being the most brilliant.In 1852, the crets like Halley? . .. comets returned on schedule, butnow were about 2,400,000 km (1,500,000 miles) apart.The year One possible answer to these questionswas pro- 1866 was to be a particularly favorableone for vided by the modern Dutch astronomer JanOort viewing the comets, and astronomers awaited their, in 1950. He suggests that there existaa giantcloud return with great anticipatiorr. But, alas, they have of literally billions of comets completelysur- never been seen again. . rounding the solar system ata distance from the sun at least as great as 150,000 times theearth's Those of us sitting on the edges ofour chairs distance.This distance is a large fraction of the awaiting the return of Comet Halley should take average distance beiween the sun and other nearby cognizance of Comet Ensor whichwas discovered stars.Every few million years, C.Ae of the nearby in 1906, and Comet Westphal whichwas discov- stars, in their random notions throughspace will ered in 1913. Both cometswere predicted to be pass close enough( to the sun to perturb a number spectacular when they passed perihelion, based on of comets in the cloud. Some of these will beIorn their orbits and on early. observations. However, out of the solar system, and will fly off intospace both comets grew very rr idlyas they approached never to be seen again. However, some will be sent, the sun and as they grew .,tiey became increasingly in toward the inner solar system. Thesecomets diffuse and faint, until they completely fadedfrom will become long period comets, with periods ofa view'. By the time the comets should have passed few million years. A number of thesecomets will perihelion they were nowhere to beseen.No pass near the massive planet Jupiter oneir trips doubt, in each of the cases we have described,we through the inner solar system.The ombined were seeing the end of the lives of the comets. In effect of Jupiter over severalpasses can to slow the case of Comet Biela, it had probably sublimed the comet's motion, in whichcase it can gradually unevenly, leaving a distorted nucleus that could become a short period comet, forever remaining in not hold together, After all,a snowball doesn't the inner reaches of the system. have muclimechanical strength. One fascinating aspect of Oort's theoryis the In June 1058 the Florentine astronomer G. B. thought that the comet cloudmay actually be Donati &Covered a comet which appearedas a frozen chunks of the nebula out of which ;thesun faint pot of light. Inwas not until late August of and planets were formed, in whichcase comets that year that the comet showeda 4211, and then it consist of the primordial material of thesolar sys- was only as long as the dianieter of the full moon. tem. What better reason that this dowe have for During September the comet increased remarkably the careful scrutiny of any comet?The hypo- in brightness as it approached thesun, and reached thetical cloud of comets is now referred toas the greatest brilliance in early ! October.The comet Oort cloud in honor of Dr. Oort. then had a tail which stretched one-fourth of the sway across the sky, was very bright and easily vis- lible to the unaided eye.After the comet passed C1METS OF THE PAST perihelion, it moved very far south in the sky and could only be followed by southern hemisphere Many comets that have been observedover the last observers.It remained visible to large southern few centuries have exhibited unexpected behavior telescopes until March 1859. in various ways, and others have been spectacular because of their great size or brilliance. Lets look Comets that are bright enough to beseen in broad at some of the unusualcomers of the past, since daylight are few and far between. Estimatei of the their behavior may tell us something of whaeto number range around four or fiveeach.century. expect from Comet Halley and other bright comets The great comets of 1843 and 1882were both day- that may appear in the near future. light comets. If one wouldscreen out f '6 full glare with a hand, one could easily hay, m these Among the most unusual occurrences in the annals cpmets. Interestingly, both comets fall intoa class of cometary study is the behavior exhibited by known as "sun grazing" comets.The comet of Comet Biela. This comet, which hadan orbital rev- 1843 passed only 120,000 km.(80,000 miles) above olution periodrof 6.75 years, was observed forsev- the surface of the sun and the, comet of, 1882 eral passes in the late 18th and 19th centuries, passed 'about 480,000 km (300,000 miles) above 18 29 4

-0 I I I I ------1 0 0\ 0 0 0 0 0 CYGI 0 o TAURUS 0 0 PLEIADES al CARIES PEGASUS 0 +100 / PISCES N70 0' 0

EQUATOR 1 AI

-10

CAPRICORNUS

lk

MAGNITUDE SCALE O 1st a O 2nd 3rd 4th

1h 6h 5h 4h 3h 2h Oh 23h. 22h

\'nenterfold \

30 '46

riLYRA CORONA o- \\ I BOREALIS HERCULES!- -

LEO 0N, .

OPHIUCHUS

VIRGO

et.

CORVUS "_..fr HYDRA SAG lwTTARIUS 0- -;* SCORPIUS

tgh 18h 17h 16h 18h 14h 13h 12h 11h loh

IkR CHART" the solar surface. These distancesseem like a come that predicted the end of the worldas the gas fortable margin until we rememberthat the sun is mixed into our atmosphere. Thenewspapers were 1,400,000 km (864,000 miles) in diameter,so in filled with stories related to the impendingpassage fact the distances are small comparedto the size of of the comet. One entrepreneur down inTexas the sun. The comet of 1882was torn apart by the sold so-called comet pills tosome of the unsuspec- huge gravitational field of thesun; it was seen as ting locals.The pills, which turned out to bea four chunks after perihelionpass. The brightest harmless mixture of sugar and quinine,were sup- sun grazing comet of the 20th centurywas Comet posed to ward off the evil effects of thecometary Ikeya-Seki observed in 1965. gasses. Business was brisk, and the fraudulent pill peddlers made a good piece of change, beforethey Comet Howard-Koomens-Michels didthe sun were caught: 1When the event actually happened, grazers one better.It plowed into thesun on there was no noticeable effect, other than the August 30, 1979. The cometwas discovered by meteor shower, mentioned earlier. an instrument flown on an Air Force satellite to study the sole corona. The conictt wasseen on ReCently, ah interesting type of cometaryevent several frames approachingvery close to the sun, has been explained; a sn-called disconnectionevent then it disappeared.At the moment of its dis- or DE in which a con one tail and grows appearance, a portion of the corona increased another. The event sees. Aleur wilt., a comet measurably in brightness.One supposes that as crosses a region in span ere the interplanetary the comet completely sublimed in the sub'sultra- magnetic field rapidly nges direction, and a hot atmosphere, the material briefly increasedthe process kno as reco. .ion causes the tail to density and therefore the brightness ofthe corona. break off. A disconne, event was observed to occur in Comet Kohoutek in 1974, and then simi- lar events were sought in other comets. A number COMET HALLEY IN 1910 of DEs have been found, including five inComet Halley in 1910. Figure 15 showsone of the events. Comet Halley was redise eredon its way toward The unusual appendage to the comet's tail its 19 ob- passage near the sun' on September 11, served half way down the tail is actually theold 1909, ughly six months before it passed peri- tail that has been disconnected from thecomet. helion.Contrast this with the upcomingpass, when itas rediscovered 40 Months before pal-. helion.This remarkable difference is due to the PLANS TO OBSERVE COMET HALLEY tremendous strides in technology in the 75or so years between passes. One hopes that the discov- Theteis a major worldwide effort underway to ob- eries during the rest of the 1985-86pass are as serve Comet Halley. Three different missions will much an improvementover 1910 as the discov- encounter the comet in March 1986. The European eries to date. To give some idea of whatwe expect Space Agency is mounting a mission called eat() to see in 1985 and 1986, we will takea brief look in honor of the great Italian painter_ whoproduced at what happened in 1910.May 1910 was the a masterful frerco bf Comet Halley after its 1300 most impressive time for Comet Halley. Thenit return. One of the chief objectives of the GiOtto was at its closest to the earth, and actually passed mission is close-up imaging of the nucleus ofthe between the earth and thesun. On May 18, the comet. .Images of the type the mission planners tail of the comet was 120° long, its greatestlength envision will go a long way toward proving or'dis- in 1910. Barnard described the comet duringthis proving Whipple's dirty iceberg model of the period as quite bright to the nakedeye, with a nucleus. Giotto will also study thegas and dust in .bluish white color. . the vicinity of the comet, and willmeasure its mag- netic field. The Giotto spacecraft willpass within On May 18, the comet passed directbetween the 1000 kilometers of the comet's nucleus inMarch earth and the sun. At the time of tactual transit 1986. The Soviet Union and Japanare each plan- of the solar disk, it was daylight inurope, and de- ning missions to the comet. The Soviet'sspace- tailed observations were madea e Moscow craft, built around their spacecraft that .flewto Observatory.The observations were completely Venus, will fly to within 10,000 km of the comet's negative.There was no sign of the cometas it nucleus.They call their mission VEGA. The passed across the sun. Given the geometry of the Japanese Planet A Mission will pass within 100,000 pass, the nucleus would have been seen in silhou- kilometers of the nucleus, also in March 1986. ette against the sun if it had beenas large as 100 km. As we said earlier,we now think that the The United States is planning several efforts.The nude% is a mere 5 km in diameter. Thevery ten- ASTRO mission consists of a group of instruments uous coma caused no observable effect. designed to carry mit-astronomical observationsin the ultraviolet reg an of the spectrum from the An interesting aspect of the passage of thecomet Shuttle bay. NASA has decided to adda pair of between the sun and earth is the ft that the tail small, wide fielcameras to the payload to obtain passed over or near the earth. Theolecule cyano- images of the entire comet. The Astropayload gen, CN, is a deadly poison, and thre were those will be sent into orbit for abouta week when

32 19 r

Figure 15. Comet Halley (top to bottom) on Jurfe 8.66,1910 from Wisconsin, on June 6.77, 1910 from Hawaii, and on June 7.29, 1910 from Lebanon. (Yerkes Observatory photograph.) 20 33 Comet Halley is closest to the earth,giving us our world.As the sun rises at observatories in the first detailed ultraviolet studies ofthe comet to Southwestern U.S., for instance, it is still darkin compliment the studies done by thespacecraft Hawaii. Thus Hawaiian observers mentioned above. can continue ob- serving the comet for a number of hours.When the sun rises in Hawaii, it is nightonce again in In additiori to the Astro Mission, NASA isplan- Europe, and so forth. In the network to take ning the International Halley Watch large (IIIW). The scale photographs of the comet to studysuch IHW will organizea series of ground-based ob- things as DEs, there are about 90 observatories serving networks to study .the comet.The basic spreact around the world (Figure 16). Sincethere idea of the networks isas follows.Studies of Comet Halley in 1910 are so many observatories, it is also very likely that and studies of other at West one of them will have clear skies whenit is comets as well showed that thereare things dark. The bottom line is, at least happening in the comet that cannot one observatory be fully ob- ,in the network will be able to observethe comet at served from one site. A completedisconnection any time. The wide-field imaging .network is only event, for instance, takes about 24hours. One one of several networks. night is too short to completely Others will carry out follow a DE, but spectroscopy, infrared and radio observations,pho- so much happens during the 'daylight hours be- tometry, and so forth. The data gathered bythese tween two nights that it is hard to correlatewhat networks will be essential to the understandingof one seer on one night with what was seen the night the data collected by the spacecraft that willstudy before. The only answer is to observethe comet the comet. We will see some for 24 hours. This very exciting observa- can be done by setting up a net- tions come from these activities. I can't waitto see work of cooperating observatoriesall, around the the close-up images of the nucleus.

INTERNATIONAL HALL WATCH

.__ 0 30 60 90 120 160I - . _ dP.. .. +30°,--- .... +300

0 \ A .1...., 1,,, qv,t 0 .

50° 300

1111111d11IIIIII 1 150 120 90 30 60 IIIIII120 150

HALLEY BAY, ANT.

Figure 10. Observatories cooperating In the Large-ScalePhenomena Network of the international HalleyWatch. 34 21 SELECTEDREADINGS

Brandt, John C.:Comets: Readings from ScientificAmer FranciWo: W. H. Freeman and Com- pany, 1981.This book is a compendium of articles about comets from Scientific American.It is appro. priate for a well read high school studentor above.

Brandt; John C. and Chapman, Robert D.:Introduction to Comets.New York: Cambridge University Press, 1981.This isa professional level book about Comets aimed at advanced undergraduate students, profbssional astronomers, and advanced amateur astronners.

Brown, Peter L.:Comets, Meteorites and Men.New York: Tap linger, 1974.

Calder, Nigel:The Comet is Coming: e Feverish Legacy of Mr. Halley.New York: Viking Press, 1981. 4 , i 4.. Chapman, Robert D., and Brandt, John C.:The Comet Book.Boston: Jones and Bartlett Publishers, 1984. This book, to be published in late 1984, isa popular level book that should be accessible by a wide range of readers. Ik

Muirden, James:The Amateur Astronomers Handbook.New York: Harper ana Row, 1983.

Norton, Arthur P.:Norton's Star Atlas.Cambridge, Mass.: Sky Publishing Corp., 1978.

Roth, G. D., ed.:Astronomy Handbook.1%1v York: Springer!Verlag, 1975.

Wilkening, Laurel L.ed.:Comets.Tucson, AZ: U. of Arizona Press, 1982.. Thisbook iscompen- dium of technical articles on comets written by the experts in the fieldand skillfully edited by Wilkening.

Yeomans, Donald K.:The Comet Halley' Handbook: An Observers Guide.Second Edition. Pasadena, CA: NASA Jet Propulsion Laboratory, 1983.This guide contains a wealth of information for the potential serious observer of the comet. Of special'note is the extensive ephemeris for CometHalley.

35

22 Part II EDUCATIONAL ACTIVITIES

The first part of this book described the nature of in which the comet has been observed during its comets, with particular emphasis on Comet Halley. pass near earth. The appearance of Comet Halley in the sky will offer a unique learning experience for studentsat Table 1 all levels. In this part of the book,we will .suggost List of Years when Comet Halley was Observed from Earth a number of activities that you can use to take maximum advantage of the opportunity. Thesug- B.C. gested activities covera wide range of skills in 239 86 11 science and mathematics. Same of the.activities are quite appropriate for primary grades, and A.D. ... others require the skills learned in the secondary 66 141 218 295 374 grades. Clearly, you the teacher will have to de- 451 530 cide which of the activities is appropriate foryour 607 684 760 students. In 'some cases, it will be possiblefor you 837 912t 1066 1145 1222 to adapt an exercise to your specific needs. The 1301 1378 material is meant toserve as a guide and as a help 1456 1531 1607 to you. Be creative in its use. 1682 1759 1835 1910 1986 Some of the activities can be done in the class- room. These activities are includes to providean As an activity, have your students be chroniclers of understanding of the historical importance of the time1Either individually or as agroup exercise, comet, and to enhance the student's understanding have the students imagine that theyare reporters of these wonderful celestial objects.However, it is onboard Comet Halley.They should report the essential that the students be encouragedto get news of planet earth as viewed from the comet for outdoors to observe .the comet first hand, whenit one particular passage. The categoriesthat can be is visible.Several activities are included that will included in the news account dependson the age introduce the skills and knowledge requiredto group of the students. Some examples of the type locate and observe Comet Halley. Comet Halleyis of information that students might includelare: the an ideal; subject for bulletin board displays in population of earth; explorations being made at the schools, by the way. time; major modes of transportation; the geography of the known part of the planet; art, music and lit- erature of the time period; scientific discoveries; IN THE CLASSROOM wars and political issues; feelings about the comet; and other major issues of the time. During the span of almost 2100years during which Comet Halley has been observed to return from the The students might report theirnews through the depths of space to pass by the sun,we have come a newspaper format, radio shows, or where equip- long way in our understanding of comets from ment is available, over closed circuit television. ancient times when cometswere considered to be When everything is complete, you might havea vapors burning in the upper atmosphere, to mod- community open house to view ancr hear about the ern times with our capability to send space mis- history of the earth as viewed from Comet Halley. sions right up to the comet. The study of Comet Hal* has givenastronomers valuable insights in Newspaper Accounts the past, and will continue to provide unique infor- mation about the nature of comets. The newspapers of May 191.0 containeda great deal of information about Comet Halley because Every return of Comet Halley to the vicinity of that month the earth was supposed to fly through earth has been recorded since 239 B.C., withone the comet's tail. Banner headlines at the time in- exception. We know ofno records of the comet cluded such comments as thisone from the May from the 163 B.C. return.Each time the comet 18,1910 New York Times. passes the earth it finds the planet in a slightly dif- ferent state; after all, great changescan take place CHICAGO IS TERRIFIED in our civilized society in just 76years. Just think Women are Stopping up Doors.and about the tremendous technological changes that Windows to Keep Out Cyanogen have taken place since Comet Halley's last visit in 1910. Comet Halley would find differences inthe Have your students do a library search of hometown size of the population, advances in medicine and newspapers to find out local reaction to Comet, science, and additions to the great creations of Halley.It would be easiest to limit the search to music, art and literature.Table 1 lists the years May 1910, when a lot of comet activity occurred.

23 36 Oral History numerous poems and works of literature that mention Comet Halley. There are still people alive today whosaw Comet Halley during 1910. Haveyour students do oral Your students could follow in the path of the great history interviews with individuals whosaw it. An masters of the past and compose works of art in- announcement over the local radio or television spired by the comet. Have your students writea stations or a request in the localnewspaper will poem or compose a song about Comet Halley. help the students locate possibleresource persons Have them make a painting or drawing of the comet to interview. as they observe it.They could design a T-shirt with the comet on it, or sculpta pager mache Have your students record their interviewson a model of it. Perhaps you couldeven sponsor an art tape recorder.It is 'important to prepare for the show todisplay various Comet Halley pieces interview. Beforehand, make sure the students created by your students. have their questions written out. Such questionsas the following might be included:How old were Comets and Life Sciences you when you saw Comet Halley? Wherewere you living at the time? What aresome of the things you There are some articles in the literature today that remember about Comet Halley? Was therea lot of speculate on the role of comets in spreading life excitement associated with the return of Comet throughout the universe. Some scientists havesug- Halley?What were some of the reactions to it? gested that a rudimentary form of life might form Itow did people learn aboutComet Halley since in the nucleus of a comet, which would then col- there was no radio or television to tell them about lide with a planet planting the seeds for the forma- it? Were you afraid? tion of life. There have even been suggestions that a comet collided with the earth and led to the ex- Time Capsule tinction of the dinosaurs. In this case, the collision would have produced a giant cloud of dust which After doing the exercises suggested to providea would have blocked sunlight and cooled the earth, historical perspective about Comet Halley, the affecting the dinosaur's food supply. These ideas students will be aware that changesoccur very provide excellent research topics foryour students. rapidly on earth. To get ready for the nextpassage, Have them survey the literature on the topic and have your students prepare a time capsule. The decide for themselves if there might beany basis time capsule might contain student predictions of for the speculation. Have your students speculate happenings on earth during Comet Halley's next on what would happen if a comet as large as Comet return in 2062. You might even plan to bury the Halley were to collide with earth today,.Such a time capsule on February 9, 1986, the date when collision is highly unlikely, but not impossible. the comet is at its perihelion point. The timecap- What is the likelihood that sucha collision would sule could then be opened on the perihelion date in affect populated areas of the planet? 2062.. Students could be guided to predict such things as:average life spans; the population of Comet Hall of Fame their city; science advances at the time; whatwe will be doing in space; and majorconcerns of the Over the past 21(70 years, there have beena num- citizens of their city, the U.S., and the world in ber of individuals who have contributed signifi- 2062.' cantly to our understanding of comets. Haveyour students do a "Comet Hall of Fame." Whenpos- As an alternative to the time capsule, your students sible, students should include a picture of thatper- could publish their predictions in the localnews- son as well as a biographical sketch and a descrip- paper so that some futire student doing a litera- tion of the contribution to comet science for each ture search in 2062 will discover this information. contributor. Some of the "Hall of Famers" might include: Aristotle., Isaac Newton, Johannes Kepler, Co e Artistically Speaking Edmund Halley, Tycho Brahe, John Winthrop, Friedrich Wilhelm I3essel, Jan Oort, Fred Whipple, In 1910 ceras were used for the first time to and E. E. Barnard. photographomet Halley.Before 1910, artists rendered Comet Halleyon several different occa- sions. For instance, it was depictedon the famous OBSERVING TIE COMET Bayeux Tapestry that highlighted the victory of William the Conqueror in 1066 at the battle of As discussedearlier Comet Halley will not be Hastings. _Art historians believe that Comet Halley nearly as spectacular in 1985-86asit was in served as the model for the Star if Bethlehem in 1910. However, with patience andcare the comet Giotto's "The Adoration of the Magi." Thisspec- can be observed.It will be at its brightest iiroun4 tacular work of art is one of the works in the December 1985 and again in April 1986.It will fresco cycle executed by Giotto di Bondone in the not be easily visible from large cities particularly in Scrovegni Chapel, in Padua.There have been the Northeastern U.S.If you live in that area of 9 24 / 37 the country, you will have toget out into the same formula will not work for both pre-perihelion country away from city lights andair pollution. and post-perihelion magnitudes, But the effort will be worthevery bit of the effort. Figure 17 is a star chart showingthe path among Pre-Perihelion: the stars. This chart, along with Figures9, 10, and 11 should be sufficient to permityou to locate the Total Apparent Magnitude comet in the sky. = 5.47 + 5.0 x log(A) + 11.1 x log(R) Brightness Post-Perihelion: Everyone has noticed howsome stars appear Total Apparent Magnitude brighter than others. The brightnesses of stars are = 4.94 + 5.0 x log(A) + 7.68 X log(R) described by their apparent magnitudes.The mag- nitude scale used in astronomy hasdeveloped over In these formulas the symbol the years into a well defined, but A stands for the dis- somewhat arbi- tance from the earth to the cornet, whichwe call trary scale. The brithtest stars in thesky are more the geocentric distance, and thesymbol R stands or less first magnitude, and the faintes( stars visible for the distance from thesun to the comet, which to someone with good eyesightand ideal observing we call the heliocentric distahce. The magnitude conditions (no clouds, city lights, or air pollution) depends on both distances fora simple reason. is sixth magnitude. The brightera star, the lower The distance from the the numerical value of its magnitude. sun determines how much The brightest light reaches the comet, and thedistance from the starssin the constellations Orion, Bootes,and Lyra earth to the comet determineshow much of the (Rigel, Arcturus, and Vega)are extremely bright._ light reflected by the comet reaches These stars are zero magnitude us. Hidden in objects. They are the constants are assumptionsabout the reflectiv- the first stars to be seen in theevening sunset and ity of the comet. Included in the last to be seen in the morning the 5.0 multiplying twilight.First the logarithm of A and theconstant multiplying magnitude stars are alsovery bright. The brightest stars in Scorpius, Cygnus, and the logarithm of R is the fact thatthe brightness of Virgo (Antares, an object decreases with the inversesquare of its Deneb, and Spica) area few examples: The second distance. magnitude stars suchas Polaris, the pole star, are moderately bright and'an be easily identified. In Table 2 we list the geocentric Third magnitude stars and heliocentric are still fainter. On a misty distances of Comet Halley at selectedtimes. The night these are usually the fainteststars that one following example for October15, 1985, when can see. The fourth magnitude starsare visible on A = 1.59 and R. = 2.16 shows how a ..moonlit or hazy night. students can cal- Fifth and sixth magni- culate total magnitudes for thecomet. tude stars are visible only under themost ideal con- ditions. If you are used to the kindsofskies you Total Apparent Magnitude see around cities, iyou can be confused bya very clear sky.If you were to go toa high mountain- = 5A7 + 5.0 x log(1.59) + 11.1 x log(2.16) top in the Southweston a superclear night there = 5.47 + 5.0 x 0.201 + 11.1 x 0.344 seem to be so many stars in the sky that it takesa minute or so to find the constellations.Figure 18 = 5.47 + 1.005 + 3.707 is a sketch of the little Dipper,with the magni- tudes of the stars indicated.Since the stars are = 10.2 magnitudes 2, 3, 4, and 5, it makesa good refer- ence in the...sky. You can also get a feeling for the The formulas are not sufficiently viewing conditions precise to.permit on a given night by checking us to calculate magnitudes to more thanone figure the Little Dipper.If you can see all the stars after the decimal point. Have clearly, it is a fairly good night. your students calcu- late some magnitudes from the datain Table 2. We have given magnitudesas checks for your students, The following formulas will letyour students pre- or for you to use if this exercise is too advanced dict the total magnitude of Comet Halley.Here we for your students. must be careful with our understanding ofwhat magnitude means. The comet is nota point object From January through April,Comet Halley will like a star, but is spread outover a larger area. The have an apparent magnitude in therange 4 to 5. total magnitude is the brightnessthe comet would So beware of the fact that it will have if the light were concentrated into require ideal con- a starlike ditions to observe the comet. Havethe students image. You can imagine that ifwe were to spread compare the apparent magnitude of Comet Halley out the light of a starover a comet sized areit with the stars in the Little Dipper. would seem fainter. A comet that How can we is third maghi- compensate for the fact that the starsare points of tude will be harder tosee than a third light and the comet isan extended object? One star, for instance. The formulaswere determined way is to observe the stars with binoculars that from the behavior, of Comet Halley at itsprevious have been purposely racked out of focusto give passes; that is, they are empirical formulas.The extended star images. 38 25 .50' T T «oo' 1 1 i 1 1----i I. i 1 1 i ®, , 40 or) CI ANDROMEDA 0 r 'NO.-CYGNUS > \_____..1 440' 0 4b/ N .01.YRA T L1RUS 0 I 110C°R1AIILAIS 1,180.07;\ HERCULES\ PL EIAOES 1 \ \N. \ 0.ARIES 4 20 PEGASUS \ %Nov 111, / \ Ne A Lto d. +M ekt'.r IA 1 X / 10 1 0.., \ . PISCES af ----__. +100 O \ I OPHIUCHUS 0 111UATDR -,, eLa..---... N sw/ -10 VSE -too CAPRICOR \ / \\L11RA M. NII . 20' \., N./

NYORA -10 MOWN! SCALE -30* 6 IN -10 0 Ind MA .0* 41, -50 Apt. 10 -eft

1 1 1 I I 1 1 I 1 I I I I i I I 6" 5^ 4" 3" 2" 1" Oh 23" 22" 21" 20" to' le" 17" 10" 10" 14" 13" 12" II" 10"

Figure 17. Prkth of Comet Halley between November 1985 and May 1986.

39 40

copy 101,10 Pm 5 Table 2 Selected Ephemeris of Comet Halley R.A. DEC DATE ,, A R MAO H M o I A.U. A.U. o 0 6 3 6 10 +13 53 7.99 7.06 21.9 3.1 ,tr 88 6 37 +13 49 7.31 6.53 co 9 1 6 45 37.5 5.4 +13 27 6.81 6.34 58,2 r.4 10 1 6 48 +12 7.8 50 6.07 6.09 86.1 9.4 11 1 6 41 +12 14 5.29 5.81 12 1 6 20 +11 118.0 8.7 54 4.67 5.56 151.7 4.8 1 1 5 47 +12 03 4.33 5.28 to 2 1 5 16 162.3 3.2 co +12 41 4.35 4.77 126.2 cc 3 1 4 58 +13 39 4.55 9.2 1.4 4 4.73 94.3 12.1 1 4 53 +14 55 4.79 4.42 16.0 5 1 5 00 +16 62.8 11.6 12 4.89 4.12 15.7 35.5 8.2 6 1 5 15 +17 22 4.78 3.79 ii2 7 1 5 33 15.3 10.5 2.8 co +18 15 4.44 3.47 14.7 15.3 0 8 1 5 54 +18 53 3.81 4.5 ,-I 1 3.10 13.8 40.3 12.2 9 6 11 +19 21 2.98 2.73 10 1 6 13 +20 12.7 66.1 19.8 00 2.04 2.34 11.1 94.6 25.2 10 15 6 03 +20 38 1.59 2.16 loco 11 11 1 10.2 110.8 25.6 5 24 +21 50 1.07 1.92 8.8 0 11 15 4 00 +22 02 136.8 20.7 '-' 11 20 0.74 1.72 7.4 170.1 5.7 3 11 +20 50 0.66 1.65 7.0 11 25 2 14 +18 172.8 4.3 20 0.62 1.57 6.6 154.5 15.7 12 1 1 07 +13 51 0.63 1.48 6.4 131.7 to 125 0 28 +10 34 0.67 29.7 co 1.42 6.3 117.6 37.9 12 in 23 48 +06 51 0.73 1.35 6.2 a',-1 12 15 23 18 +03 102.1 45.7 12 20 49 0.82 1.27 6.2 j 68.8 50.8 22 54 +01 26 0.92 1.19 6.1 12 25 22 36 -00 24 i, 77.5 53.7 1.02 1.11 6.0 67.6 54.7 1 1 22 17 -02 23 1.16 1.01 co 1 15 21 a'''' 5.8 55.3 53.4 co 48 -05 13 1.50 0.80 5.1 a, 2 1 21 18 33.7 43.0 ,-I -08 27 1.56 0.62 ., 4.1 10.2 26 21 09 -09 34 1.56 0.59 16.5 2 15 20 53 -11 ; 3.9 6.5 10.9 61 1.50 0.60 4.1 14.7 24.8 3 1 20 28 -16 12 1.27 to 35 0.72 4.4 34.7 51.3 20 21 -17 42 1.18 0.77 4.4 0 3 10 20 11 40.7 56.6 ,-I -19 52 1.06 0.84 4.5 48.4 3 15 20 GO -22 30 0.94 0.92 61.6 3 20 19 44 -25 4.5 56.6 64.8 51 0.81 0.99 4.5 65.8 66.1 3 25 19 22 -30 15 0.69 4 1 1.07 4.3 76.5 85.0 18 24 -38 45 0.53 1.18 4.1 45 17 22 *-44 96.0 57.5 14 0.46 1.24 4.0 110.6 49.1 48 16 16 ( -47 06 0.43 1.29 4 10 15 24 -47 3.9 123.1 40.7 32 0.42 1.32 4.0 131.5 34.8 4 12 14 32 -46 24 0.42 1.35 co 4 14 13 4.0 139.2 29.1 co 44 -43 53 0.43 1.38 4.2 ca 4 16 13 03 -40 28 145.2 24.6 - 4 18 0.45 1.41 4.4 148.7 21.7 12 31 -36 43 0.48 1.44 4.6 4 20 12 06 -33 00 149.3 20.9 0.52 1.47 4.8 147.7 21.4 4 25 11 24 -25 to 06 0.64 1.54 5.4 139.3 25.1 5 1 10 56 -18 2,1 0.80 coQ, 1.63 6.1 128.5 28.9 5 15 10 31 -10 32 1.24 1.84 ,-I 6 1 7.4 108.7 31.4 10 26 -06 45 1.80 2.08 8.7 7 1 10 34 -05 10 90.5 29.3 2.74 2.47 10.2 64.5 21.7 8 1 10 52 -06 03 to 3.57 2.86 11.2 39.9 13,1 9 1 11 11 -08 00 4.17 3.20 °° 10 1 12.0 18.2 5.6 cc 11 27 -10 21 4.52:. .-I 3.57 12.5 16.7 4.6 11 1 11 39 -12 54 4.62 3.91 12.8 12 1 11 42 -15 06 40.0 9.4 4.50 4,22 13.0 67.4 12.5 1 1 11 31 -16 30 4.27 4.53 13.1 c.- 2 1 11 05 99.4 12.4 co -16 07 4.10 4.83 13.3 133.9 Cl5 3 1 10 34 -13 48 8.5 ,- 4.16 5.10 13.5 158.1 4.2 4 1 10 05 -10 03 4.57 5.38 5 1 9 49 -06 13.8 141.2 6.7 53 5.21 5.65 14.3 110.8 9.6 NOTE: R.A. and DEC are the rightascension and declination of Comet Halley, using the equinox of distance from Comet Halley to the earth, the date. A is the the geocentric distance. R Is the distancefrom Comet Halley to thesun, the heliocentric distance. MAO 'is the totalmagnitude. 6 lithe aniribetween thesun and the comet as seen from earth, and iris the atiiTt eWeen thesun and the earth as seen from the comet.

27 Al 3 number). Under ideal conditions, a pair of 7 X 50 5 binoculars should letus observe Comet Halley when it has a magnitude of 10.8. 4 Length of Comet Halley's Tail in Miles Measuring angles in the sky is not very difficult. 2 4 There are several good angle reference points in the sky. For instance, the Moon is about one-half degree in diameter. The pointer stars in the Big 5 (Variable) Dipper are 5° apart, while the stars on the top of the bowl of the Dipper are 10° apart (tre Figure 20). A paper clip held at arms length can be used like calipers to measure angles in the sky as shown in Figure 21. A cross staff can also be constructed to measure the length of the comet's tail (or other *4 large angles on the sky) in degrees, with moderate precision. To construct a cross staff, carry out the following steps. / .1. Select a straight length of 1" by 1" lumber, at least 36" long. 2 2. Decide on one of the 1" sides to be the top, Figure 18. The Little Dipper showing the magnitudes then mark off 1" increments along each side of the stars. of the stick.Starting at either end, number the marks 1", 2", 3", and so on.

With a wide angle telescope or a pair of binoculars, 3. Drive two nails into the top of the stick,one students will be able to see Comet Halley before it at each end to serve as sights. becomes visible to the unaided eye. Figure 19 is a graph of the limiting magnitude for various objec- 4. Select a seco9d piece of 1" by 1" lumber, at tive apertures (the diameter of the main lens of the least 21" intiength to be the cross piece. telescope or binoculars), given in inches. A 7 X 50 Carefully find and mark its center. Then pair of binoculars has an objective of 50 mm 26.4 mark 1" increments on the sides, beginning mm/inch or 2 inches (rounded to the nearest whole at the center. Number these increments 1", 2", and so on, beginning at the center and working outward toward each end.

8 5. live a nail at the 2" mark, the 5" mark, and the 10" mark at each side of the center. 7 6. Construct a slide mechanism for the cross piece. The simplest way is to drivea nail on 6 the bottom of the cross piece 1-1/2" from the center on each side of the center. Bend the nails toward the center to serve as a slide. t 5

4 Figure 22 shows a cross staff built to these specifi- I t cations.Notice that it has a slightly better slide 1 mechanism made from two blocks of 1" by 1" 3 lumber. The cross staff is easy to upe. Hold it just i as the young lady is doing in the figure, and align 2 the sight nails on the long piece with one end of .the comet. Then slide the cross piece backward or 1 forward until one of the nails (the 2", the 5", or fi the 10") on the cross piece is aligned with the other end of the comet. Read two numbers from r------' 0 the cross staff: the distance of the cross piece from 8 7 9 10 11 12 13 14 your eye, and the distance from the center cis the MAGNITUIN cross piece of the nail that you used for the other Figure 19. A graph of the limiting magnitude versus end of the comet. Table 3 can be used to convert objective size in inches for telescopes or binoculars. these measurements to degrees.

28 42 00

ti

Figure 20. The Big Dipper showing anglesceles.

Figu!21. Using a paper clip heldat arms length to compare known angularextents in the sky with unknown extents.

s , JY L_

t

'. I

Aim the cross staff at the head and tail of the comet to determine the angularmeasurement of Cornet Halley. Figure t2. Using a well made cross staff to Measure angles, The construction ofthe cross staff is reasonably clear in the photograph.

43 29 Table 3 The students should be encouraged to make draw- Conversions for Cross Staff Measurements ings of the comet as acctirately as possible. The Width nucleus (if one is visible), coma, and tail of the comet should be recorded for each observation. Length 2 inches 5 inches 10 inches The other data required on the form should also be (inches recorded. When possible, students should compare Angle in Degrees unaided eye views of the comet with those made through a telescope or binoculars.Have the stu- 2 45.0 68.2 78.7 dents list advantages of viewing the comet with the 3 33.7 59.0 73.3 telescope or binoculars.Have them determine 5 21.8 45.0 63.4 some of the advantages of viewing the comet with- out use of a telescope or binoculars. 8 14.0 32.0 61.3 12 9.6 22.6 39.8 Students should be encouraged to phOtograph 15 7.6 18.4 33.7 Comet Halley when they have the appropriate 18 6.3 15.5 29.1 equipment. The camera with standard lens must be mounted on a tripod and fast color or black and 21 5.4 13.4 26.6 white film must be used: The students should ex- 24 4.8 11.8 22.6 periment with different exposure times.They 27 4.2 10.5 20.3 might try a sequence of shots like 5, 20, 40, and 80 seconds.Longer exposures will smear the 30 3.8 9.5 18.4 comet due to the earth's rotation. 33 3.5 8.6 16.9 36 3.2 7.9 1G.5 If your school has a block driven telescope, it might be used as a base for the camera. The camera should be fastitriled (tape will be fine) to the for- Once the observed length of the tail is measured in ward part of he telescope.With a clock driven degrees, its length in kilometers, miles or astronom- telescopelonger exposures can be made. If a black ical units can be calculated. Remember that one and -white photograph is being made during a time astronomical unit is 150,000,000 kilometers or when the moon is out or during twilight, a red 93,000,000 miles. filter might be used. Increase your exposure times by a factor of four to compensate for the filter. Length of tail Try several exposures of different lengths. Use the = geocentric distance x photographs to determine the magnitude of Comet sin (observed length in degrees) Halley from the stars. The magnitude of stars can be determined from various observer handbooks. Suppose that on January 15, 1986, your students 40 observe Comet Halley to have a 45° tail. Look up Plotting the Location of Comet Halley the geocentric distance in Table 2, and find the sine of the tail length in a convenient trig table. Then' A familiarity with the use of a star chart and knowledge of how to plot the locations of Comet Length =1.50 x sin(5°) Halley are needed to successfully "keep track" of = 1.50 x 0.087 = 0.13 astronomical unit. Halley. A star chart is included to assist you in this activity.See center of booklet and Figure 17. The toof the comet does not lie exactly in the plane of the sky.It may also have extent toward Notice on the star chart that the constellations are or away from you. Have your students discuss the named.Constellations are "pictures" in the sky length you calculate in light of this fact. To com- and are used to identify the different sky regions. pensate for the projection effect, you can divide The different sized dots represent the different the length, calculated above by magnitudes of the stars. (Refer to the magnitude key on the star chart.) Also, notice the line that sin(sunrcomet-earth angle in degrees- runs through the center of the star chart in an east-s observed comet tail length in degrees) west direction. This line is celestial equator. The celestial equator is nothing more than the earth's where the sun comet-earth angle is the angle[5in equator projected onto the plane of the sky. Table 2. To find a location on earth, one uses longitude and Recording Observations Q fatitude; however, different termsare used in refer- 0 ence to analogous coordinates in the sky. In the It is very portant for the students to record their sky, right ascension (RA) is the coordinator equiva- obseryons. A suggested Comet Halley observa- lent to longitude and declination iequIvalent to tion orm is included.Careful recording of their latitude. Notice on the star chart that at the bot- ob ations will permit students to compare one tom are a series of numbers 6, 5,...,0, 23,...9. ob .rvation with another. Each number is the right ascension of that area of

30 44 COMET HALLEY OBSERVATIONFORM

Name

Date Time of Observation

ObserVer Location

Weather Conditions , ,

Moon Phase

Comet Halley's Location in Sky: ,

Degrees Above Horizon Constellation Region

S4tch Area of Sky

e,

*

,--

45 31 PHYSICAL APPEARANCE OF COMET FORM

UNAIDED EYE VIEW TELESCOPE/BINOCULAR VIEW .

Size of Objective

Magnification

Length of tail in degrees Length of tail in degrees

Color of Comet Color of Comet

Estimated Magnitude Estimated Magnitude i

Description of Nucleus, Description of Nucleus, Coma, Tail Coma, Tail

. .

Drawings (Comet shape, Drawings (Comet shape, direction, length of tail) direction, length of tail)

.

32 46 the sky.Right ascension is measured eastward numerals refer to huurs of right from the Vernal Equinox along the ascension. celestial equa- Review the plotting exercise ifnecessary. tor, In hours. Every degree moved eastwardfrom the Vernal Equinox equalsan addition of four 2. Draw a line from 58° through the minutes of right ascension. Therefore, 15° center of east of the circle to 238°. At the 58° point in the the Vernal Equinox is equalto one hour of RA; earth's orbit, Comet Halley will 30°/ east equals two hours of RA, pass from eta. south to north across the earth'sorbital plane. On a star chart one uses It will later pass from north to the a "+" sin to represent lo- south of the earth's orbital planeat the 238° cations north of the celestialet, uator and a "-":. point. sign for locations south of the celestialequar. An object with the declination of +15° 5° 3. A slit needs to be made in the earth'sorbit north of the celestial equator, while -15°ould so that you can" insert Comet Halley's orbit place the object 15° south of thecelestial equ r. later. Table 2 lists selected RA and declination Comet Halley's orbit is positioned locatis such that it extends 1:85 A.U. from thesun for Comet Halley from 1981-1987.A more inc sive list can be found in on the 58° side of the earth's orbit and 0.85 an ephemeris, such as that A.U. on the 238. side. Onour scale this cor- in the Comet Halley Handbook(see Bibliography). responds to a distance of five and As an activity, have the studentsplot the positions one-half for Comet Halley on a starehart. inches and two and one-half inches.Using a Generally, the single edged razor blade, makea slit in the planets and the moonmove from west to east earth's orbit according to the among the stars. This is not sip for Comet Halley. instructions. Halley moves from east to west. 4. Comet Halley's orbit is ellipticalin shape. Refer to the drawings in Figure 25. To plot Halley's positions, the students Note are to put a the terms on the drawing: semimajoraxis, point at the intersection of the RA anddeclination semiminor axis, semilatus rectum, aphelion, locations for each of the dates given.Notice that and perihelion.Also, note the equations Halley/ motion among the stars isnot a smooth given to solve for the 'differentpart of line frOm east to west. Halley'sapparent motion Comet Halley's orbit, ifyou would like to appears to loop occasionally. This isa result of the assign this as an exercise. earthpassing Comet Halley duringour orbit around the sun for that particular gear. Again, a For Comet Halley, .the length of thesemiminor axis general astronomy textbook willprovide a more is equal to a distance of 17.945 A.U.The eccen- detailed explanation of retrogrademotion. tricity is equal to 0.9672671.Ifave the students solve the different equations togilta better under- Once the plotting exercise is completed,students standing of Comet Halley's orbit, if theyare a the can use their star charts to assist them in locating appropriate level of mathematics. Comet Halley either withan optical instrument or the naked eye. The star chart willbe especially Let a = semimajor axis= 17.945 A.U. helpful from November 1985through May 1986. Let e = eccentricity = 0.967281 Comet Halley's Orbit Then: Earlier we discussed the elements ofthe orbit of a comet. To supplement this information,have your Perihelion Distance = a(1-e) students construct a model of Comet Halley'sorbit. Figure 23 gives an overview of theangular elements = 17.945 x (1.0-0.967281) of Comet Halley's orbit,near the earth's orbit. = 17.945 x 0.032719 = 0.587 A.U. 1. Construct the earth's orbit.On a piece of poster board roughly 24" x 24" drawa circle Aphelion Distance = a(l+e). with a diameter of 6 inches. In thiscase 3" equats one astronomicalunit. The sun is lo- = 17.945 x (1.0+0.967281) cated at the center of the circle.Then draw another circle with a diameter of 12inches = 17.945 x 1.967281 = 35.3029 A.U. to provide a'2 A.U. scale, anda circle with an 18" diameter to provide a 3 A.U. scale. Length of Semiminor axis= a(1-4 )1/2 Next divide the earth's orbit into12 equal parts.Each part is to represent the earth's = 17.945 x (1.0 - 0.9672812)1/2 location for one month of theyear. Label the earth's orbit as indicated in Figure24. = 17.945 x (0.06436746)1/2 This is the view you would have ifyou were 17.945 x 0.2537 located above the earth's orbit andwere looking down on it. In the figure theRoman = 4.552 A.U.

33 47 PERIHELION

.:

LOCATION OF VERNAL EQUINOX--

9 EARTH'S ORBIT COMET 9.6LLEY'S ORBITo-

Figure 23. Angular elements of Comet Halley's orbit. The quantitiesare as follows: A is the longitude of the ascending node, with a value 58.15°. B is the inclination of the orbitto earth's orbital plane, with a value 162.24.C is the -4gument of perihelion and D is the perihelion point.

XDZ 1800 210°

yz 2 01° 7 I Mar 27041411,4411 i90° VT 300° W XX * 111 0°EL 330° ,30° XX11 0° IL

Figure 24. A view of the earth's orbitseen from above.The right ascension is marked both in hours (Roman numerals) and in degrees. The three circles represent the earth's orbit (1 A.U.) as wellas a 2 A.U. and 3 A.U. scale. The heavy line marks the cut to insert the model of Comet Halley's orbit. 34 48 DISTANCE FROM SUN TO COMET HALLEY COMET, HALLEY

LENGTH OF SEMIMINOR AXIS = all-e2)%

PERIHELION DISTANCE all -e)

PERIHELION a- APHELION

1 SEMILATUS RECTUM LENGTH OF SEMILATUS RECTUM = e2)

Figure 25. The geometry of Comet Halley'sorbit. All relevant terms are definedon the figure.

Length of Semilatus Rectum= a(1-e2) is a good resource to help you withyour comet ac- tivities. Pe -haps such a club could hosta star party. = 17.945 x (1.0 - 0.9672782) Work with your school librariannow to begin to = 17.945 x 0.064367 = 1.155 A.U. gather boc ks and articles for the studyof comets. Watch for radio or televisionprograms that could Have your students onvert theiranswers to miles be of interest and value toyour students. You or kilometers. might want to publicize your aLtivitieson the media. 5. Draw Comet Halley's orbitto scale.Note that the distance of the semilatusrectum is Computing an Ephemeris for Comet Halley 1.115 A.U. (you can haveyour studentil cal- culate this number, too). The total distance A table of positions of the comet atvarious times across the orbit atthe sun is twice this dis- is called an ephemeris. Ifyour school has a com- tance or 2.230 A.U.Since we are using a puter that operates in .the BASIC language,your scale of 3" equals one A.U., the latusrectum students can calculate an ephemerisfor Comet is 2.230 x 3 = 6.69 inches. To makematters Halley using the program startingon page 37, easier, you can round this off to 6 and 3/4 written by R. Chapman. It is notas sophisticated inches.Similar,calculate the perihelion as programs written by astronomers who calculate distance in inch .In round numbers, it will the position of the comet toan accuracy of sec- be 1 and 3/4 inc es. Drawan elliptical arc onds of arc, but it will allowyou to calculate its *representing a portion of the orbitas shown position to better than a few minutes ofarc.It in Figure 26a. We will not draw the entire also prints* out the position of thesun and the 'orbit; it would be excessively largeon our comet's magnituee. The X, Y, and Zcoordinates scale. of the sun and Comet are rectangularcoordinates in a right handed system where the x-axispoints 6. Cut out the orbit and slide it throughthe slit toward the vernal equinox, and the xy-planeis the from the underside of the earth's orbit. Use plane of the earth's equator. The rightascension, Figures 26b and 26c to properly orientthe declination X, Y, and Z are referredto the vernal comet's orbit. equinox at 1950. R is the heliocentricdistance of the comet, and delta is its geocentric distance.The Places to Visit- Things To Do program also calculates the altitude and azimuth of the comet for a given time. These numbersare Field trips to a planetariumor an observatory are a good for your latitude, and the timethat you must during your study of Comet Halley. Resource input. These numbers will helpyou locate the persons from the community should be contacted comet. An altitude larger than 90°means the to visit your classroom. Your localastronoiy club comet is below the horizon.The details of the

36 49 (a)

FEBRUARY 9, 1986 DECEMAER23, 1985

NOVEMBER 9, 1985 MARCH 10, 1986

MARCH 30, 1986

(b)

/./

162°

(c)

Figure 26. (a) Measurements for a portion of Comet Halley's orbit; (b) Location of Comet Halley on five dates, to be used in the model; (c) Tilt of Comet Halley's orbit to the earth's orbit.

36 50 program are beyond the scope of this booklet,and Apple IIe computer using BASIC,but it should might be a good library project foryourstudents. work on other systems. We donot endorse any The program has been annotatedwith REM particularpersonalcomputer; we are merely statements to help see what is goingon.The providing a warning that theprogram has not been author of the program wrote it andtested it on an checked out of a variety ofsystems.

Even though Comet Halley will fadeout of sight in the visible sky soon after April 1986,not to reappear for another 76 years, the young people that you have worked withwill carry with them memories of Comet Halleyall through their lives. In addition, they will gainthe skills, understanding, and enthusiasm to look at the skies for theirentire lives. Whocan predict the total life-long impact ofsuch a meeting with Comet Halley?

PROGRAM 10 REM *** INPUT NUMERICAL CONSTANTS*** 20REM *** OB = OBLIQUITY 30 PI 3.14159265:0B 0.48914:RD 57.2957795sTPI 2.0 * PI 40REM *** INPUT ORBITAL ELEMENTS ANGLESIN RADIANS*** 50REM EECCCENTRICITY; 010NODE LONGITUDE;02PERIHELION LONGITUDE 60 REM ININCLINATION; A=SEMI MAJOR AXIS;PPERIOD IN YEARS 70 REM PDJULIAN DAY OF PERIHELION;PHADDITIONAL DAY FRAC. AT PERI. 80 E 0 0.9672768,01 1.81482798102 1.95211743 90 IN = 2.83160961:A= 17.941104:P 75.993 100 pp 2446478.5IPH 0.45174 110 GOSUB 6000 120 N I TP / P / 365.25: REMMEAN MOTION 121 HOME s VTAB 8: PRINT "THIS PROGRAMCALCULATES INFORMATION" 122 PRINT "OF INTEREST FOR COMETHALLEY FOR ANY" - 123 PRINT "LOCATION ON EARTH. THE OPERATION" 124PRINT "IS SELF EXPLANATORY.",PRINT 125PRINT "SOME DEFINITIONS: ":PRINT "X,Y,Z ARE COORDINATES AL" IN EQUATOR' 126PRINT "SYSTEM IN A.U. R IS DISTANCE FROM SUN.". 127 PRINT "DELTA IS DISTANCE FROMEARTH."s PRINT 128PRINT "INPUT YOUR LATITUDE IN DECIMALDEGREES": INPUT " NEGATIVE IF. IN SOUTHERN HEMISPHERE ";LA 129 LA LA / RD 130 INPUT "DATE OF INTEREST? (MM,DD,YYYY)" ;MM,DD,YYsS = 1: GOSUB5000 140 INPUT "TIME OF INTEREST? (HH,MM)";HH,M1 150 JH 0I;(HH + M1 / 60) /24 160 M ((JD PD) + (JH PH)) * N 165 OMB 7500: REMFIND POSITION OF SUN 170 KY 10 8 180GOSUB 7000: REM SOLVE KEPLER'SEQUATION 190 SOWS 80001 REM FINDX,Y,Z OF COMET 193 REM SCSUNCOMET DISTANCE ess sc SCR (XC * XC + YC * YC+ ZC * ZC) 210 X XC + XSsY YC + YSsZ = IC + ZS 220REMCALCULATE EC-EARTH COMET DISTANCE 230REM ACRIGHT ASCENSIONOF COMET; DC0DECLINATION OF COMET 240 EC SQR (X *X+Y*Y+Z* Z) 243 IF KY 1 THEN 250 248 DM 0.003772 * EC * NsM M DMsKY = is GOTO 180 250 SA Z / ECsCA MIR (1.0 SA * SA)s GOSUB 8500 260 DC A3sSA Y / (EC * CA):CA = X / (EC* CA), GOSUB 8500 270 AC A3 280REM THANGULAR SEPARATION SUNCOMET 290 CA SIN (DC) *SIN (OS) +COS (AS AC) *COS (DC) * COS (DS)

51 37 300 SA mm SOR (1.0 CA * CA). GOBUB 8500 319 TM mm A3 320SOSUB 9000e REM FWD SIDEREAL TIME 330 HA m ST AC. REM A:04HOUR ANGLE 332 IF HA ( 0 THEN HA mm HA + TP .$40 X.s COS (DC) * SIN (HA) 359 Y SIN (DC) *COS (LA) COS (DC) *COS (HA) * SIN (LA) 360 Z SIN (DC) *SIN (LA) +COS (DC) * COS (HA) * COB (LA) 370 SA ZiCA SOR (1.0 SA * SA)1 BOGUS 8500 380 EL mm AMA X / CAMP Y / CA1'008U8 8509 385IF EL ) 4.72 THEN EL EL TP 390 AZ gm A3 400. IF JD ) 2446470.5 THEN 430 410 MT 5.47 + (5.0.*LOG (EC) + 11.1 *LOG (SC)) / 2.3026 420GOTO 440 .439 MT mg 4.94 + (5.0 * 'LOG (EC) + 7.68 * LOG (SC)) / 2.3026 '440 MN 14.1. + (5.0 * LOG (EC) + 5.0 *LOG (SC)) / 2.3026 2000HOME I PRINT " POSITION OF COMET HALLEY ": PRINT 2919PRINT "DATE: "OM;"/";DD;"/";YY;" JULIAN DAY " ;JD: PRINT 2020 RA mm 3.81971863: REM CONVERT RADIANS TO HOURS 2022 ST mm ST 41, RA:H INT (ST)0612 INT t60 * (ST H)) 2024PRINT "LOCAL TIME: SOLAR ";HH;" ."01;" SIDEREAL= " ;H ;" "02 2026PRINT 2028PRINT "COORDINATES OF THE SUN: 2029 XS - INT (10000 * XS) / 10000sY8 mm INT (10000 * YS) / 10000sZ8 INT (10000 * Z8) / 10000 2030PRINT " 1mm ";XS;" Y "044F, Zim 2040 AS mm AS * RA:H INT (AS):M2 gm INT (69 * (AS H)) 2050 DS mm DS * RDel) mm INT (DS):M3 INT (fte * (DS D)): IF D ( im 90 THEN 2970 2060 D D 359:M3 mm 60 M3 2070PRINT " R.A.- " ;H ;" " ;M2 ;" " ;D ;" "03 2075PRINT edPRINT "COORDINATES OF THE COMET. 2078 XC ft INT.(10000 * XC) / 10900:YC mm INT (10000 * YC) / 10000:ZC - INT (10000 * ZC) / 10000 2980PRINT " X= " ;XC ;" Ymm " ;VC ;" ";ZC 2090 AO AC * RAIH mm INT (AC):M2 mm INT (60 * (AC H)) 2100 DC mm DC * RDiD mm /NT.-(DC):M3 INT (60 * (DC D)). IF D ( mm 90 THEN 2120 2110 D 359.013 mm 60 M3 2120PR NT " R.A. ms" ;H ;" " ;M2 ;" DEC- " ;D ;" " ;M3 2130PRT Rom "; INT ( 100 * SC + 0.5) / 100; 2140PRINT " DELTA- "; INT (100 * EC + 0.5) / lea 2145 TH mi INT (190 * TH * RD + 0.5) / 100 2150PRINT " SUNCOMET ANGLE ";TH 2160 HA gm HA * RAIIX gm 9: IF HA )12 THEN X mm 1:HA mm 24 HA 2170 H INT (HA).012 mm INT (60 * (HA H) ) 2180PRINT " HOUR ANGLE " ;H ;" " ;M2; 2190 IF X 0 THENPRINT " WEST" 2800 IF X 1 THENPRINT " EAST" me AZ mm AZ * RDeD mmINT (AZ):M3 - INT (69 * (AZ D)) 2229PRINT " AZIMUTH- " ;D ;" " ;M3 223$ EL mo EL * RD.D mm INT (EL):M3 mm INT (60 * (EL D)) 2240PRINT " ELEVATION " ;D ;"" ;M3 ene MT mm INT (10 * MT) / 100N mm INT (10 * MN) / 10 2260PRINT : PRINT se NUCLEAR MAGNITUDE- " ;MN 2270PRINT," TOTAL MAGNITUDE- " ;MT 2280PRINT 4990PRINT "DO YOU WANT ANOTHER DATE? (Y/N): ": GET C$ 4992 IF C$ mm "Y" THEN 130 4993HOME : vTps 12e PRINT " SIGNING OFF" 4999END 5999 REM **CALCULATES JULIAN DAY**

5010 IF MM ( mm 2 THEN X mo1

38 52 5020IF MM ) 8 THEN X 0 9 5030 IF S 9 THEN C = 2 5040 IF S = 1 THEN C = INT ((YY - X) / 100) 5050 Al -INT ( INT (365.25 * (YY- X)) - C) 5060 8 0INT (367 * ((MM- 2) / 12 + X)) 5070 IF 8 ( 0.THEN = 8 + 1 3080 JD 1721088.5 + DD + Al +B 5090RETURN 6000REM 6095 REM*** SETS UP VECTORS TO CONVERTFROM ORBITAL *** 6010 REM*** TO ECLIPTIC COORDINATES *** 6015 REM sees PX - COS (02) * COS (01) -SIN (02) *SIN(01) * COG (IN) 6030 PV -COS (02) * SIN (01) +SIN (02) *COS(01) *COB (IN) 6040 PZ SIN (02) it SIN (IN) 6050 GX - - SIN (02) *COS (01) COS (02)*SIN (01)* 6060 GY =- SIN (02) * COB (IN) SIN (01) COS (02)*COS (01)* COS (IN) 6070OZ - COS (02) *SIN (IN) 6890RETURN 7880REM ************************* 7010REM * ITERATIVESOLUTION OF * 7920REM* KEPLER'S EQUATION * 7930REM******4****************** 7050TPt = 2.0 * PI 7960X 0 M / TPI 7070M 0 (X - INT (X)) * TPI 7080EA 0 M 7099ES = EA -.(EA - M E *SIN (EA)) / (1.0 E * COS (EA) ) 7100 IF ASS (ES - EA) / ES ( 1E- 8 THEN 7130 7110EA 0 ES 71e.OOTO 7090 7130RETURN 7580REM POSITION OF THE SUN 7510 REM91 -MEAN ANOMALY; OP-PERIHELIONLONG; Li -SUN'S LONGITUDE 7529DI (JD - 2415020) + JH 7530X1 - (358.4758 + 0.9836* DI) / 360. 755091 = (X1 - INT (X1)) * 360 / RD 7560OP - (281.2208 + 4.7E- 5 * D1) / RD 7578 LI = 01 + 0.03344 * SIN (611) + 3.49E- 4 *SIN (2 * Gl) + OP 7580X1 = Ll / TPI 7585REM CALCULATE FINAL LI AND PRECESS 7586 REM THEN SUN AND COMET ARE REFERREDTO SAME EPOCH 7590 LI (X1 - INT (X1)) * TPI + 2.437E- 4 * (2433282.5 - JD) / 365.25 7f.Vb REM BE -SUN -EARTH DISTANCEIXS,YS,IS=RECT.COORD. OF SUN 7600 SE - 0.99972 / (1.0+ 0.01675 *COS (LI - OP)) 7610 X8 0 SE *COS (LI) 76e0 YS - SE *SIN (Li) *COS (OS) 7630 ZS - SE *SIN (Li) *SIN (08) 7648REM --- AS -RIGHT ASCENSION OF SUN-- 7643REM--- DS- DECLINATION OF SUN- -- 7650 SA se ZS / SEICA-SQR (1.0 - SA * SA): (3OSUB8500 7960 DS 0 03e8A- YS / (SE.* CA)NCA - X8 / (SE* CA)* GOSUB 8500 7670 AS 0 A3 7880RETURN 8000REM CALCULATES COORDINATES IN 8010REM ECLIPTIC SYSTEM xl as A * ( COS (EA)- E) 8W30 YI 0 A *80R (1 - E * E) *SIN (EA) 8048 X 0 X1 * PX + Yl.* GX 8050 Y X1 * PY + Y1 * GY 8060 Z 0 X1 * PI + Y1* OZ 8063 REM***ROTATE TO EQUATORIAL SYSfiEM 8070 XC 0 XsYC - Y * COS (DB) Z * SIN (OB)sZC =Y* SIN (MB) + Z * (08) COB 53 39 aim RETURN 8500REM 8585REM TWO ARGUMENT ARCTAN 8510 REMCAmCOS(A);SAmSIN(A); RETURNS A IN PROPER QUADRANT 8515REM 8520 IF ASS (CA)( lE 8 THEN 8540u 8530 A3 -ATN (SA / CA): GOTO 8560 8540IF SA ) 0 THEN A3 sa 0.5 * PI 8550IF SA (0 THEN A3 - 1.5 * PI 8560IF CA (8 THEN 8590 8570 IF CA ) 0 AND SA ( 0 THEN 8600 8580SOTO 8610 8590 A3 m A3 + PI: GOTO 8610 8600 A3 m A3 + 2.0 * PI .8610 RETURN 9000REMCALCULATE SIDEREAL TIME 9010 T1 - (JD 2415020) / 36525s REMCENTURIES SINCE 1900 9020 XI m (18.64606 + 2400.0513.* TI) / 24 + 0.5 + JH 9030 ST - (X1 INT (X1)) * TPA REMSIDEREAL TIME IN RADIANS 9040RETURN 10000 12 m ((1986 + YY) / 2 1900) / 100 10010 X3 as 3.07234 + (0.00186 * Te):Y3 m 20.0468 (0.0085 T2):Z3 m Y3 / 15 10020 13 - (JD PD) / 365.35 10030 X4 m 7.2722E S * T3 * (X3 + (Z3 *SIN (AC) *TAN (DC))) 10049 X5 m 4.8481E 6 * T3 * Y3 * COS (AC) 10050 AC m AC +X4tDC - DC + X5 10060RETURN

54

40 INDEX

amateur astronomers, 3 constellation, 25, 28, 30 ammonia, 10 Copernicus, N.,1 aphelion, 33, 35 cross staff, 28-30 Aristotle, 1, 24 cyanogen, 19, 23 Astro Mission, 19, 21 declination, 27, 30, 35 BASIC program, 37-40 dinosaur, 24 Bayeux Tapestry, 24 direct motion, 8 Chapman, R., 36 disconnection event, 19, 21 comet, eccentricity, 5 brightness, 9, 18 ephemeris, 5, 27, 35 -40 collision with, 24 field trips, 35 coma, 9, 10, 16 Galileo, 1 diameter, 9 generatrix, 5, 7 composition, 10 geocentric distance, 25, 27, 36 discovery, 3-4 Giotto, 19, 24 hall of fame, 24 Giotto Mission, 19 heal, 9 gravity, 1, 19 hyarogen cloud, 15 Halley, E., 1, 5, 24 lifetime, 16, 18 heliocentric distance, 25, 27, 35 naming, 3 Herschel, C. 5 nucleus, 9, 10, 15, 16, 21 horizon, 13, 14, 15, 35 nature, 10 hydrogen cyanide, 10, 19, 23 orbit, 1, 6-9 Ikeya, K., 4 parts, 9 inclination, 5, 8 tail, 9, 16, 23 International Halley Watch, 21 dust, 15, 16 interstellar space, 5 length, 9 Kepler, J., 1 orientation, 10, 16 latitude, 35 plasma, 16, 16 magnitude, 25, 26, 27, 28, 30, 32 Comet Bennett, 15 limiting, 28 Comet Biela, 18 meteor, 15 Comet Donati, 18 meteor shower, 15, 16 Comet Ensor, 18 methane, 10 Comet Halley, 1, 11, 10, 20, 29 methyl cyanide, 10 appearance, 9-10 Mitchell, M., 5 art, 24 molecules, brightness, 25, 29 daughter, 10 classroom activities, 23-24 parent, 10 ephemeris, 27, 36-40 Newton, I.,1, 3 historical importance, 1-3 Oort, J., 18 lifetime, 18 Oort cloud, 18 newspaper accounts, 23 oral history, 24 observations, 30, 31, 32 orbit 1, 5, 8 orbit, 3, 33-36 elliptical, 1, 5 path, 26 parabolic, 1, 6 photography, 30 hyperbolic, 5 plans to observe, 19-21 orbital elements, 5; 8, 34 position, 12, 13, 14, 26, 27, 30 parallax, 1, 2 recovery, 4 perihelion, 3, 5, 10, 24, 33, 35 returns, 23 I1errine, C., 4 tail, 30 perturbation, 18 encounter with, 23 photon, 15 viewing, 10, 24-40 Pons, J., 4 Comet Honda-Mrkos-Pajdusakova, Ptolemy, C., 1 Comet Howard-koomens-Michels, 19 radical, 16 Comet Ikeya-Seki, 4, 19 retrograde motion, 8, 9 Comet Kohoutek, 5, 6, 10, 18, 19 right ascension, 27, 30, 34, 35 Comet IRAS-Araki, Alcock, 6, 9, 10 Seki, T., 4 Comet Schwassmann-Wachmann 1, 5 semimajor axis, 33, 35 Comet Schwassmann-Wachmann II, 6 semiminor axis, 33, 35 Comet Tago.Sato-Kosaka, 16 semilatus rectum, 33, 35 Comet West, 1, 16 Seneca, L., 1 Comet Westphal, 18 solar wind, 15 comets, sublimation, 10, 16, 18 daylight, 10, 18 time capsule, 24 longperiod, 8, 18 Tycho Brahe, 1 short-period, 8, 9. 18 vernal equinox, 3, 8, 33, 34, 36 sun-grazing, 18, 19 water, 10 sun-hitting, 19 Whipple, F., 10. 24 cornet seeker, 3 conic section, 5, 7 circle, 7 ellipse, 5, 7 hyperbola, 5, 7 parabola, 1, 5, 55 41